Fix infinite looping and crash problem.

MALLOC_DEBUG is complaining about corrupted memory on taskprocessor
listeners, but I'm not seeing it, nor is valgrind. Not sure what's up
there.



git-svn-id: https://origsvn.digium.com/svn/asterisk/team/mmichelson/threadpool@377377 65c4cc65-6c06-0410-ace0-fbb531ad65f3