TCP/UDP/SCTP: fix Dereference of null pointer found by Clang Analyzer

The warning coming after change in g018b84de8

Change-Id: Ia96cdb2993a6283d8de2647c6723ec6b2b0977eb
Reviewed-on: https://code.wireshark.org/review/3752
Reviewed-by: Evan Huus <eapache@gmail.com>