Make sort_alphabet() not fail too badly with characters in [128,255].

Treat all values in the alphabet as unsigned.

Change-Id: I4476c75352f32673a9cd131ea233465f3376fa25
Reviewed-on: https://code.wireshark.org/review/4747
Reviewed-by: Guy Harris <guy@alum.mit.edu>
(cherry picked from commit a517d500b4d5bdedba68127d295a37c2dbe4bf2c)
Reviewed-on: https://code.wireshark.org/review/4748