bridge_native_rtp: Fix possible segfaults on leaves/joins

bridge_native_rtp: Fix possible segfaults on leaves/joins

native_rtp_bridge_get can return any result from the ast_rtp_glue_result
enumerator and the join/leave functions for bridge_native_rtp seem to assume
that if the result wasn't local that it was remote. Meanwhile forbid can be
returned by that function which can mean certain glue pointers are NULL. Then
when the join/leave functions try to use members of that pointer, boom.
Segfault.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@391430 65c4cc65-6c06-0410-ace0-fbb531ad65f3