Fix Heap-buffer-overflow READ in Assimp::MD5::MD5MeshParser::MD5MeshParser

[sashashura]

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=54435
Closes #4922

The buffer read overflow happens because the string szStart used in AI_MD5_PARSE_STRING_IN_QUOTATION doesn't contain double quotes. In this reproducer case for exampel it is \U00000013\U00000013\U00000013\xecΐ\xec\xec\xec\xec\xec\xec\xec\xec\xec\xec\xec\0.
However it is guaranteed to be zero terminated:

	elem.szStart = buffer;

	// terminate the line with zero
	while (!IsLineEnd(*buffer))
		++buffer;
	if (*buffer) {
		++lineNumber;
		*buffer++ = '\0';
	}

[kimkulling]

Looks fine, thanks for the fix.

[kimkulling]

Merged, thanks a lot for your contribution.