Merge bitcoin#18413: script: prevent UB when computing abs value for num opcode serialize

2748e87 script: prevent UB when computing abs value for num opcode serialize (pierrenn)

Pull request description:

  This was reported by practicalswift here bitcoin#18046

  It seems that the original author of the line used a reference to glibc `abs`: https://github.com/lattera/glibc/blob/master/stdlib/abs.c

  However depending on some implementation details this can be undefined behavior for unusual values.

  A detailed explanation of the UB is provided here : https://stackoverflow.com/questions/17313579/is-there-a-safe-way-to-get-the-unsigned-absolute-value-of-a-signed-integer-with (by [Billy O'Neal](https://twitter.com/malwareminigun))

  Simple relevant godbolt example :  https://godbolt.org/z/yRwtCG

  Thanks!

ACKs for top commit:
  sipa:
    ACK 2748e87
  MarcoFalke:
    ACK 2748e87, only checked that the bitcoind binary does not change with clang -O2 🎓
  practicalswift:
    ACK 2748e87

Tree-SHA512: 539a34c636c2674c66cb6e707d9d0dfdce63f59b5525610ed88da10c9a8d59d81466b111ad63b850660cef3750d732fc7755530c81a2d61f396be0707cd86dec

Author: fanquake

src/script/script.h

@@ -331,7 +331,7 @@ class CScriptNum
 
         std::vector<unsigned char> result;
         const bool neg = value < 0;
-        uint64_t absvalue = neg ? -value : value;
+        uint64_t absvalue = neg ? ~static_cast<uint64_t>(value) + 1 : static_cast<uint64_t>(value);
 
         while(absvalue)
         {

src/test/fuzz/integer.cpp

@@ -141,11 +141,7 @@ FUZZ_TARGET_INIT(integer, initialize_integer)
 
     const CScriptNum script_num{i64};
     (void)script_num.getint();
-    // Avoid negation failure:
-    // script/script.h:332:35: runtime error: negation of -9223372036854775808 cannot be represented in type 'int64_t' (aka 'long'); cast to an unsigned type to negate this value to itself
-    if (script_num != CScriptNum{std::numeric_limits<int64_t>::min()}) {
-        (void)script_num.getvch();
-    }
+    (void)script_num.getvch();
 
     const arith_uint256 au256 = UintToArith256(u256);
     assert(ArithToUint256(au256) == u256);

src/test/fuzz/scriptnum_ops.cpp

@@ -123,10 +123,6 @@ FUZZ_TARGET(scriptnum_ops)
                 script_num &= fuzzed_data_provider.ConsumeIntegral<int64_t>();
             });
         (void)script_num.getint();
-        // Avoid negation failure:
-        // script/script.h:332:35: runtime error: negation of -9223372036854775808 cannot be represented in type 'int64_t' (aka 'long'); cast to an unsigned type to negate this value to itself
-        if (script_num != CScriptNum{std::numeric_limits<int64_t>::min()}) {
-            (void)script_num.getvch();
-        }
+        (void)script_num.getvch();
     }
 }