2 critical but easy to fix integer overflows

[dns43]

decodeGeneric() in lodepng.c:4812 has integer overflow.
lodepng_inspect_chunk() in lodepng.c:4716 has integer overflow.

Can be fixed easily by adding check if INT_MAX-chunkLength-12 < chunk, or using static int lodepng_addofl(size_t a, size_t b, size_t* result) in lodepng.cpp

Happy to provide PoC