res_pjsip_refer: Handle INVITE with Replaces failure after answer.

* Fixed hangup handling of the session->channel after answer if the
ast_channel_move() or ast_bridge_impart() fails.  We are still the thread
controlling the session->channel so we need to call ast_hangup() to kill
the channel.

* Fixed debug messages in refer_incoming_invite_request() referencing
incorrect channnels on success.  Code comments now say why the
session->channel cannot be used.

Review: https://reviewboard.asterisk.org/r/4422/
........

Merged revisions 431956 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@431957 65c4cc65-6c06-0410-ace0-fbb531ad65f3