chan_sip: Fix deadlock of monlock between unload_module and do_monitor

Release monlock before calling pthread_join.  This ensures do_monitor
cannot freeze by locking monlock during module unload.

(closes issue ASTERISK-21406)
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/3284/
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Merged revisions 410224 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 410225 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@410226 f38db490-d61c-443f-a65b-d21fe96a405b