Squelch a warning.

In at least some versions of GLib, g_array_free() warns if passed a null
pointer, rather than just silently returning.

Change-Id: I1bfc0a81faa1eeebe288f6e0cc58ebfb64784958
Reviewed-on: https://code.wireshark.org/review/3068
Reviewed-by: Guy Harris <guy@alum.mit.edu>