bridge_channel: Don't queue unmapped frames.

bridge_channel: Don't queue unmapped frames.

If a frame is written to a channel in a bridge we
would normally queue this frame up and the channel
thread would then act upon it. If this frame had no
stream mapping on the channel it would then be
discarded.

This change adds a check before the queueing occurs
to determine if a mapping exists. If it does not
exist then the frame is not even queued at all. This
stops a frame duplication from happening and from
the channel thread having to wake up and deal with
it.

Change-Id: I17189b9b1dec45fc7e4490e8081d444a25a00bda