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

Change-Id: If527af69ec96a91c4127c95307c3b93b79b53e2f
Reviewed-on: https://code.wireshark.org/review/4865
Reviewed-by: Guy Harris <guy@alum.mit.edu>
(cherry picked from commit 991743b0b1cce99305a4b3162d6c520a09877c15)
Reviewed-on: https://code.wireshark.org/review/4866