Get rid of some no-longer-necessary casts.

g_ascii_isXXX() can be handed a char value safely; they won't try to use
it as an index into a table before checking it.

(cherry picked from commit e3477bd13635a637d1337093457dbad499bd5538)

Change-Id: I35aaeaabcc896adb747aad628d259dd0b5b09c2e
Reviewed-on: https://code.wireshark.org/review/4803
Reviewed-by: Guy Harris <guy@alum.mit.edu>