Fix a mmap() error scanning PDF files, as described on clamav's git repo commit
off_t is 64-bit, size_t is still 32-bit and that causes unexpected integer
promotion here:
map_off = map->len - 2048
First the unsigned subtraction is performed, and then the unsigned (!) value
is sign-extended to 64-bit. Hence a negative value becomes positive, which is
wrong.
Reported by: Franz Schwartau <franz@electromail.org>
Obtained from: https://wwws.clamav.net/bugzilla/show_bug.cgi?id=2300