Use g_ascii_isalnum() rather than isalnum().

That way, we don't have to worry about casting the argument (which, in
one place, was done wrong - casting to int preserves the sign-extension
done with signed chars), and don't have to worry about a locale in which
particular 8-bit byte values are considered alphanumeric characters.

Change-Id: I129b4bfdad70ade4ab6e0a1d2c13d59ae9e6f524
Reviewed-on: https://code.wireshark.org/review/4751
Reviewed-by: Guy Harris <guy@alum.mit.edu>