Unlock the dial operation lock on a failed dial

If a dial operation fails, the pbx_outgoing_attempt routine will exit without
first having unlocked the outgoing dial lock. This would be a "bad thing".


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@396521 f38db490-d61c-443f-a65b-d21fe96a405b