res_pjsip_refer: Fix issue where native bridge may not occur upon completion of a transfer.

There are two methods within res_pjsip_refer for keeping track of the state of a transfer.
The first is a framehook which looks at frames passing by to determine the state. The second
subscribes to know when the channel joins a bridge. In the case when the channel joins the
bridge the framehook is *NOT* removed and this prevents the native RTP bridging technology
from getting used.

This change gets the channel and if it still exists remove the framehook.

Review: https://reviewboard.asterisk.org/r/4218/
........

Merged revisions 428760 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/13@428761 f38db490-d61c-443f-a65b-d21fe96a405b