Avoid sign-extending bytes before handing them to <ctype.h> macros.

Pointed out by the Visual Studio code analyzer.

Change-Id: Idd429b4d0fb3db11ce171c3a5b38bdc55cc53c15
Reviewed-on: https://code.wireshark.org/review/2988
Reviewed-by: Guy Harris <guy@alum.mit.edu>