rtp_engine: Clean up after a failed remote bridge

Upon failure of an INVITE transaction meant to initiate a remote native
bridge, rtp_engine.c would not clean up non-reference-counted bridge
instance pointers leaving a dangling pointer which was being used to
perform a local native bridge after the other channel had hung up. This
lead to dereferencing into freed memory and plenty of AO2 errors. This
change allows the remote native bridge loop to clean up properly when
the bridge fails.

(closes issue ASTERISK-23310)
Reported by: Jeremy Laine


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@409521 f38db490-d61c-443f-a65b-d21fe96a405b