ConfBridge: Use of bridge or user profiles that don't exist

Bridge and user profiles are not checked for existence before use.  The lack
of a fully formed bridge profile can cause a segfault when sounds are accessed.
This change ensures that bridge and user profiles exist prior to usage
attempts.

Review: https://reviewboard.asterisk.org/r/1264/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@323106 65c4cc65-6c06-0410-ace0-fbb531ad65f3