PR/337: Fix incorrect handling of offsets (wrong sign)
Test:
$ perl -e 'print "ABC", "\xff\xff", "\x00"x70000, "DEF"' > abc.data
$ cat abc.magic
0	string		ABC	ABC test file format
>(3.s)	search/0x3000	DEF	\b, DEF sub type
$ file -m abc.magic abc.data
abc.data: ABC test file format, DEF sub type