Fix some bugs in the SIP tcp helper thread.

 - fix a spot where a lock wouldn't get unlocked in an error condition
 - call ast_mutex_destroy() on the lock before freeing its memory

(related to issue #11972)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@105734 65c4cc65-6c06-0410-ace0-fbb531ad65f3