Exception in the ANTLR shim

After few minutes of fuzzing mruby using the testcases from test/t/

terminate called after throwing an instance of 'std::range_error'
  what():  wstring_convert::from_bytes
==13251== 
==13251== Process terminating with default action of signal 6 (SIGABRT)
==13251==    at 0x4E7AF47: raise (raise.c:51)
==13251==    by 0x4E7C8B0: abort (abort.c:79)
==13251==    by 0x71F2256: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.28)
==13251==    by 0x71FD605: ??? (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.28)
==13251==    by 0x71FD670: std::terminate() (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.28)
==13251==    by 0x71FD904: __cxa_throw (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.28)
==13251==    by 0x71F4C0B: std::__throw_range_error(char const*) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.28)
==13251==    by 0x6D94479: std::__cxx11::wstring_convert<std::codecvt_utf8<char32_t, 1114111ul, (std::codecvt_mode)0>, char32_t, std::allocator<char32_t>, std::allocator<char> >::from_bytes(char const*, char const*) (locale_conv.h:324)
==13251==    by 0x6D94032: antlrcpp::utf8_to_utf32[abi:cxx11](char const*, char const*) (StringUtils.h:43)
==13251==    by 0x6D934D4: antlr4::ANTLRInputStream::load(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) (ANTLRInputStream.cpp:40)
==13251==    by 0x6D9322B: antlr4::ANTLRInputStream::ANTLRInputStream(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) (ANTLRInputStream.cpp:22)
==13251==    by 0x6D932CA: antlr4::ANTLRInputStream::ANTLRInputStream(char const*, unsigned long) (ANTLRInputStream.cpp:26)
==13251== 
==13251== HEAP SUMMARY:
==13251==     in use at exit: 1,082,303,564 bytes in 18,122,690 blocks
==13251==   total heap usage: 59,484,413 allocs, 41,361,723 frees, 3,887,947,618 bytes allocated
==13251== 
==13251== LEAK SUMMARY:
==13251==    definitely lost: 0 bytes in 0 blocks
==13251==    indirectly lost: 0 bytes in 0 blocks
==13251==      possibly lost: 69,776 bytes in 4 blocks
==13251==    still reachable: 1,082,233,788 bytes in 18,122,686 blocks
==13251==                       of which reachable via heuristic:
==13251==                         stdstring          : 52 bytes in 1 blocks
==13251==         suppressed: 0 bytes in 0 blocks
==13251== Rerun with --leak-check=full to see details of leaked memory
==13251== 
==13251== For counts of detected and suppressed errors, rerun with: -v
==13251== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
Aborted

[andreafioraldi]

lgtm

[h1994st]

This issue has been solved in b9b5d303902df6cd2a0f57669b2e6a824d277a6e.

I wrap the parsing function with a try/catch. A nullptr will be returned if any errors occur.

The grammar mutator will skip a test case, if there is no previously stored tree file, or a parsing error occurs.