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
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Merged revisions 410226 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@410227 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -34430,12 +34430,16 @@ static int unload_module(void)
 
 	ast_mutex_lock(&monlock);
 	if (monitor_thread && (monitor_thread != AST_PTHREADT_STOP) && (monitor_thread != AST_PTHREADT_NULL)) {
-		pthread_cancel(monitor_thread);
-		pthread_kill(monitor_thread, SIGURG);
-		pthread_join(monitor_thread, NULL);
+		pthread_t th = monitor_thread;
+		monitor_thread = AST_PTHREADT_STOP;
+		pthread_cancel(th);
+		pthread_kill(th, SIGURG);
+		ast_mutex_unlock(&monlock);
+		pthread_join(th, NULL);
+	} else {
+		monitor_thread = AST_PTHREADT_STOP;
+		ast_mutex_unlock(&monlock);
 	}
-	monitor_thread = AST_PTHREADT_STOP;
-	ast_mutex_unlock(&monlock);
 
 	/* Destroy all the dialogs and free their memory */
 	i = ao2_iterator_init(dialogs, 0);