Get rid of another toupper() call in favor of g_ascii_toupper().

(cherry picked from commit 991743b0b1cce99305a4b3162d6c520a09877c15)

Change-Id: Ib7650b773504f7f4572f297a01301bdf895ed917
Reviewed-on: https://code.wireshark.org/review/4867
Reviewed-by: Guy Harris <guy@alum.mit.edu>