fixed fuzzing crash in simplecpp::Macro::expandToken()

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/usr/bin/../include/c++/v1/vector:570:16: runtime error: reference binding to null pointer of type 'const value_type' (aka 'const simplecpp::Token *const')
    #0 0x641340cb01e2 in front /usr/bin/../include/c++/v1/vector:570:9
    #1 0x641340cb01e2 in simplecpp::Macro::expandToken(simplecpp::TokenList*, simplecpp::Location const&, simplecpp::Token const*, std::__1::unordered_map<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>, simplecpp::Macro, std::__1::hash<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, std::__1::equal_to<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, std::__1::allocator<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>> const, simplecpp::Macro>>> const&, std::__1::set<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>, std::__1::less<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>> const&, std::__1::vector<simplecpp::Token const*, std::__1::allocator<simplecpp::Token const*>> const&) const /home/user/CLionProjects/simplecpp-rider/simplecpp.cpp:1994:49
    #2 0x641340ca99ee in simplecpp::Macro::expand(simplecpp::TokenList*, simplecpp::Location const&, simplecpp::Token const*, std::__1::unordered_map<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>, simplecpp::Macro, std::__1::hash<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, std::__1::equal_to<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, std::__1::allocator<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>> const, simplecpp::Macro>>> const&, std::__1::set<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>, std::__1::less<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>>, bool) const /home/user/CLionProjects/simplecpp-rider/simplecpp.cpp:1887:31
    #3 0x641340ca4c53 in simplecpp::Macro::expand(simplecpp::TokenList*, simplecpp::Token const*, std::__1::unordered_map<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>, simplecpp::Macro, std::__1::hash<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, std::__1::equal_to<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, std::__1::allocator<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>> const, simplecpp::Macro>>> const&, std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>>&) const /home/user/CLionProjects/simplecpp-rider/simplecpp.cpp:1529:26
    #4 0x641340c74fa5 in preprocessToken(simplecpp::TokenList&, simplecpp::Token const**, std::__1::unordered_map<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>, simplecpp::Macro, std::__1::hash<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, std::__1::equal_to<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, std::__1::allocator<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>> const, simplecpp::Macro>>>&, std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>>&, std::__1::list<simplecpp::Output, std::__1::allocator<simplecpp::Output>>*) /home/user/CLionProjects/simplecpp-rider/simplecpp.cpp:3210:32
    #5 0x641340c60ba7 in simplecpp::preprocess(simplecpp::TokenList&, simplecpp::TokenList const&, std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>, std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>>&, std::__1::map<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>, simplecpp::TokenList*, std::__1::less<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>>>, std::__1::allocator<std::__1::pair<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char>> const, simplecpp::TokenList*>>>&, simplecpp::DUI const&, std::__1::list<simplecpp::Output, std::__1::allocator<simplecpp::Output>>*, std::__1::list<simplecpp::MacroUsage, std::__1::allocator<simplecpp::MacroUsage>>*, std::__1::list<simplecpp::IfCond, std::__1::allocator<simplecpp::IfCond>>*) /home/user/CLionProjects/simplecpp-rider/simplecpp.cpp:3681:14
    #6 0x641340cece61 in LLVMFuzzerTestOneInput /home/user/CLionProjects/simplecpp-rider/fuzz.cpp:31:5
    #7 0x641340ad3168 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) (/home/user/CLionProjects/simplecpp-rider/fuzz+0xd3168) (BuildId: c67f7ae0ae4f90bce06e963fc0d95c2c7f3417a7)
    #8 0x641340ad3866 in fuzzer::Fuzzer::MinimizeCrashLoop(std::vector<unsigned char, std::allocator<unsigned char>> const&) (/home/user/CLionProjects/simplecpp-rider/fuzz+0xd3866) (BuildId: c67f7ae0ae4f90bce06e963fc0d95c2c7f3417a7)
    #9 0x641340aa9a8b in fuzzer::MinimizeCrashInputInternalStep(fuzzer::Fuzzer*, fuzzer::InputCorpus*) (/home/user/CLionProjects/simplecpp-rider/fuzz+0xa9a8b) (BuildId: c67f7ae0ae4f90bce06e963fc0d95c2c7f3417a7)
    #10 0x641340ab6198 in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) (/home/user/CLionProjects/simplecpp-rider/fuzz+0xb6198) (BuildId: c67f7ae0ae4f90bce06e963fc0d95c2c7f3417a7)
    #11 0x641340a9d947 in main (/home/user/CLionProjects/simplecpp-rider/fuzz+0x9d947) (BuildId: c67f7ae0ae4f90bce06e963fc0d95c2c7f3417a7)
    #12 0x7e81557e7ccf  (/usr/lib/libc.so.6+0x29ccf) (BuildId: 0865c4b9ba13e0094e8b45b78dfc7a2971f536d2)
    #13 0x7e81557e7d89 in __libc_start_main (/usr/lib/libc.so.6+0x29d89) (BuildId: 0865c4b9ba13e0094e8b45b78dfc7a2971f536d2)
    #14 0x641340a9ff84 in _start (/home/user/CLionProjects/simplecpp-rider/fuzz+0x9ff84) (BuildId: c67f7ae0ae4f90bce06e963fc0d95c2c7f3417a7)

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../include/c++/v1/vector:570:16 in

[firewave]

This looks like something we could/should detect. I filed https://trac.cppcheck.net/ticket/12466 about it.