Fix gzip file size check

[vampirefrog]

libvgm/utils/MemoryLoader.c

Line 57 in 7b694e5

if(loader->srcSize >= 2 && loader->srcData[0] == 31 && loader->srcData[1] == 139) // check for .gz file header

Smallest possible gzip file size, according to the spec, is 18 bytes (10 byte header + 8 byte footer). Smallest I've been able to generate is 20 bytes (echo -n "" | gzip | hd). I'd make this check for 18 bytes at least. Technically you should also check the flags field and check for the extra fields too, but I think the 18 bytes check is a decent bugfix, since it would fix the line:

loader->decSize = ReadLE32(&loader->srcData[loader->srcSize - 4]);

if the file was smaller than 4 bytes.