Fix issue where an attended transfer could not be completed under a rare scenario.

Fix issue where an attended transfer could not be completed under a rare scenario.

When completing an attended transfer chan_sip does a check to make sure the extension
in the URI portion of the Refer-To header is a local valid extension. We don't actually
need to check this since we know for sure the other channel is already up and talking to
the extension. Some devices do not put the extension in the Refer-To header either, which
can cause the extension check to fail. We now no longer do this check if it is an attended
transfer.

(closes issue #14628)
Reported by: sverre
Patches:
      14628.diff uploaded by file (license 11)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@181328 65c4cc65-6c06-0410-ace0-fbb531ad65f3