Resolve a ref leak in threadpool.c

Ownership of the listener reference is not transferred because the
listener is reffed when placed into the taskprocessor. Ensure that the
listener is dereffed properly.


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@382489 f38db490-d61c-443f-a65b-d21fe96a405b