Dissection of sid28 needs null check.

Bug: 10542
Change-Id: I8fcfbbd96833af2c57754a4c8f96cf702972e22a
Reviewed-on: https://code.wireshark.org/review/4603
Reviewed-by: Michael Mann <mmann78@netscape.net>
(cherry picked from commit 6e42a729235b2e514bd9add18cfcb923cc11a883)
Reviewed-on: https://code.wireshark.org/review/4607
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>