When checking if a message is Diameter also check for a minimum message length.

This improves the detection of Diameter messages within a TCP bytestream (i.e.,
when the Diameter PDUs don't neatly align with frames).

Bug: 10362
Change-Id: I49a6e8cf076a6ab8a14761493aab9f3b11e4756e
Reviewed-on: https://code.wireshark.org/review/3557
Petri-Dish: Evan Huus <eapache@gmail.com>
Petri-Dish: Jeff Morriss <jeff.morriss.ws@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>
(cherry picked from commit 433eeda109bdd5e4ed3860d13ea67e357befae6b)
Reviewed-on: https://code.wireshark.org/review/3561
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>