Oops, I missed one "cast a char to int and use it as a subscript" case.

Casting a signed char with a negative value to int will preserve the
value, so it'll still be a negative subscript.  Cast to guchar instead,
to make sure 0x80 through 0xFF are treated as 128 to 255, not -128 to
-1.

Change-Id: I1f0b33ba3686e963d45317b45465ff335431d17f
Reviewed-on: https://code.wireshark.org/review/4742
Reviewed-by: Guy Harris <guy@alum.mit.edu>