bridge_native_rtp.c: Fix native rtp bridge data race.

native_rtp_bridge_compatible() didn't lock the bridge channels before
checking the channels for native bridging ability.  As a result, one of
the channel's native format capabilities structure got replaced out from
under the native bridge check.  Use of a stale pointer to freed memory
causes bad things to happen.

MALLOC_DEBUG, DO_CRASH, and the
tests/channels/pjsip/transfers/blind_transfer/caller_direct_media
testsuite test caught this.

* Add missing channel locking in native_rtp_bridge_compatible().

Change-Id: If25fdb3ac8e85563c4857fb8216b3d9dc3d0fa53