call_leg: local_bridge: Avoid null pointer access if CN-side not ready

This happens if for instance an HNBGW drops the RAB-AssignmentRequest
and does nothing with it.

call_leg.c:348:15: runtime error: member access within null pointer of type 'struct rtp_stream'

Related: OS#5401
Change-Id: I67d2d5b2dd3b367c34f929d63c056306ec001431
1 file changed