Check for signs in unsigned numbers and fail if we see one.

Check for signs in unsigned numbers and fail if we see one.

-1 is not an unsigned number.  For that matter, neither is +1;
"unsigned" means "without a sign", and they both have signs.

ANSI C's strto{whatever} routines - even the ones that supposedly are
for "unsigned" values - and the GLib routines modeled after them allow a
leading sign, so we have to check ourselves.

Change-Id: Ia0584bbf83394185cde88eec48efcdfa316f1c92
Reviewed-on: https://code.wireshark.org/review/17511
Reviewed-by: Guy Harris <guy@alum.mit.edu>