res_parking: Fix crash due to race condition when unloading.

There is currently a race condition when unloading the res_parking
module. Depending on the will of the universe the subscription
invocation may occur AFTER the module is unloaded. This is because
the module does NOT use stasis_unsubscribe_and_join when terminating
the subscription. It merely uses stasis_unsubscribe.

This change makes it use stasis_unsubscribe_and_join which is documented
for usage in this exact scenario.

AST-1520 #close

Review: https://reviewboard.asterisk.org/r/4375/
........

Merged revisions 431114 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/certified/branches/13.1@431116 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/parking/parking_manager.c

@@ -685,7 +685,7 @@ int load_parking_manager(void)
 
 static void parking_manager_disable_stasis(void)
 {
-	parking_sub = stasis_unsubscribe(parking_sub);
+	parking_sub = stasis_unsubscribe_and_join(parking_sub);
 }
 
 void unload_parking_manager(void)