Race condition when ISDN CallRerouting/CallDeflection invoked.

Race condition when ISDN CallRerouting/CallDeflection invoked.

The queued AST_CONTROL_BUSY could sometimes be processed before the
call_forward dial string is recognized.

* Moved setting the call_forwarding dial string after sending a response
to the initiator and just queue an empty frame to wake up the media thread
instead of an AST_CONTROL_BUSY.

* Added check for empty rerouting/deflection number and respond with an
error.

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@311297 65c4cc65-6c06-0410-ace0-fbb531ad65f3