As suggested in http://www.wireshark.org/lists/wireshark-dev/200809/msg00075.html

As suggested in wireshark.org/lists/wireshark-dev/200809/msg00075.html
(as referenced in https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=2907 ) and
https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=3411 :

Write a new convenience routine for finding a conversation and, if it is not
found, create it.  The frame number and addresses are taken from pinfo (as is
the common case).

Use this function in a bunch of dissectors.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@32790 f5534014-38df-0310-8fa8-9805f1628bb7