bridge_native_rtp: Fix crash involving masquerade

It is possible for a channel to be masqueraded out of a bridge which
means it may no longer have RTP glue to check upon leaving said bridge.
If this situation occurred (it's possible at least during dial and call
pickup) then Asterisk would crash. This change makes sure the glue is
checked before use.

(closes issue AST-1290)
Reported by: John Bigelow


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@409900 f38db490-d61c-443f-a65b-d21fe96a405b