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

Fixes Bug #10453

Change-Id: I39f64b801ce9231059ee7e03416dd3828dffeb1f
Reviewed-on: https://code.wireshark.org/review/4051
Reviewed-by: Bill Meier <wmeier@newsguy.com>