Be less lazy about disabling a Visual C++ warning.

As Graham pointed out, "#pragma warning(disable..." affects the rest
of the file. Add a push+pop so that we only operate on the line in
question. Ideally we'd be able to use "suppress" but an "#endif"
prevents that.

Change-Id: Ia01d6c245879f1c845dc68c18caea2cbceb273ef
Reviewed-on: https://code.wireshark.org/review/4569
Reviewed-by: Gerald Combs <gerald@wireshark.org>