chan_sip.c: Fix channel staging assertion failure.

The failing assertion ensures that the final snapshot gets generated so
CDR records can get finalized.  The only place where a channel staging
snapshot flag could be left set is in chan_sip.c:handle_request_bye().
The function could return before clearing the flag because the channel
could dissappear while the function had to have the channel unlocked.

* Fixed handle_request_bye() channel snapshot staging coverage area to not
have a return in the middle of it and be unable to clear the staging flag.

* Pushed the channel snapshot staging coverage area into
ast_rtp_instance_set_stats_vars() to ensure that the staging is not
interrutped.

* Made callers of ast_rtp_instance_set_stats_vars() not call it with any
channels or channel driver private locks held to eliminate the deadlock
potential.  The callers must hold references to the passed in channel and
rtp objects.

* Eliminated sip_hangup() trying to get the bridge peer.  It is futile at
this point because the channel could never be in a bridge.

Review: https://reviewboard.asterisk.org/r/3431/
........

Merged revisions 412385 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@412386 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/rtp_engine.h

@@ -1745,6 +1745,8 @@ int ast_rtp_instance_get_stats(struct ast_rtp_instance *instance, struct ast_rtp
  * \param chan Channel to set the statistics on
  * \param instance The RTP instance that statistics will be retrieved from
  *
+ * \note Absolutely _NO_ channel locks should be held before calling this function.
+ *
  * Example usage:
  *
  * \code