packet-sip.c: col_...() functions must not be called under 'if(...tree)' (even indirectly).

Fixes Bug #10453

Change-Id: Ib3e5b480284ec1649b3728263f8fe164d53059cd
Reviewed-on: https://code.wireshark.org/review/4053
Reviewed-by: Bill Meier <wmeier@newsguy.com>