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

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/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@428760 65c4cc65-6c06-0410-ace0-fbb531ad65f3