features.c: Fix lingering channel ref while Bridge() application is active.

Using the Bridge application to bridge a channel that is executing an
applicaiton such as Wait results in a lingering Surrogate channel in the
CLI "core show channels" output even though it has already hungup.

* Fix bridge_exec() to not hold onto the current_dest_chan ref once it has
been put into the bridge.

* Eliminated bridge_exec()'s use of RAII_VAR().

ASTERISK-24224 #close
Reported by: Mark Michelson

Review: https://reviewboard.asterisk.org/r/4041/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@424668 f38db490-d61c-443f-a65b-d21fe96a405b