epee: string_tools: remove dot from get_extension

Fixes a regression introduced in monero-project#9254. Previously it did not
include the dot.

Author: tobtoht

contrib/epee/src/string_tools.cpp

@@ -178,7 +178,12 @@ namespace string_tools
 
   std::string get_extension(const std::string& str)
   {
-    return boost::filesystem::path(str).extension().string();
+    std::string ext_with_dot = boost::filesystem::path(str).extension().string();
+
+    if (ext_with_dot.empty())
+      return {};
+
+    return ext_with_dot.erase(0, 1);
   }
 
 	//----------------------------------------------------------------------------

tests/unit_tests/epee_utils.cpp

@@ -1443,6 +1443,14 @@ TEST(StringTools, GetIpInt32)
   EXPECT_EQ(htonl(0xff0aff00), ip);
 }
 
+TEST(StringTools, GetExtension)
+{
+  EXPECT_EQ(std::string{}, epee::string_tools::get_extension(""));
+  EXPECT_EQ(std::string{}, epee::string_tools::get_extension("."));
+  EXPECT_EQ(std::string{"keys"}, epee::string_tools::get_extension("wallet.keys"));
+  EXPECT_EQ(std::string{"3"}, epee::string_tools::get_extension("1.2.3"));
+}
+
 TEST(NetUtils, IPv4NetworkAddress)
 {
   static_assert(epee::net_utils::ipv4_network_address::get_type_id() == epee::net_utils::address_type::ipv4, "bad ipv4 type id");