Fix some logically dead code reported by Coverity (CIDs 1158807, 1158808, 1229737).

Replace some conditional + fprintf + DISSECTOR_ASSERT()s with
DISSECTOR_ASSERT_HINT() or just change the DISSECTOR_ASSERT() to
DISSECTOR_ASSERT_NOT_REACHED() when the hint requires formatting.

Change-Id: I80c7c6579d7755a60d644b5ef60ec4e3299ba0ab
Reviewed-on: https://code.wireshark.org/review/4115
Reviewed-by: Evan Huus <eapache@gmail.com>
Petri-Dish: Evan Huus <eapache@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>