Fix issues in some ARM compilers, due to char being unsigned?

I'm not 100% certain the comparisons were right even with signed char;
make the comparisons unsigned vs. unsigned, regardless of whether char
is signed or not.  (No, C doesn't require it to be signed; that's why
there's a "signed" keyword.)

Change-Id: Icbbd1019a2f7d4ebb40d821255834f825cd7c5a7
Reviewed-on: https://code.wireshark.org/review/4731
Reviewed-by: Guy Harris <guy@alum.mit.edu>