gif: expert info for unknown data block type

gif: expert info for unknown data block type

Before this patch, the dissector would assume that any data block type
that is unknown must be the trailer byte. Check explicitly if we have a
trailer byte and bring up an expert info for unknown data block types.

Change-Id: I4c3087eb403e2e668a22628062edd0240309a2bc
Reviewed-on: https://code.wireshark.org/review/21790
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Petri-Dish: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>