add missing lock/unlock on pthread map mutex to VariableServer::shutdown fixes #1133 (#1134)

closes #1133

trick_source/sim_services/VariableServer/VariableServer_shutdown.cpp

@@ -4,10 +4,12 @@
 int Trick::VariableServer::shutdown() {
     listen_thread.cancel_thread() ;
     std::map < pthread_t , VariableServerThread * >::iterator it ;
+    pthread_mutex_lock(&map_mutex) ;
     for ( it = var_server_threads.begin() ; it != var_server_threads.end() ; it++ ) {
         (*it).second->cancel_thread() ;
         // cancelling causes each var_server_thread map element to be erased by the exit_var_thread function
     }
+    pthread_mutex_unlock(&map_mutex) ;
 
     return 0 ;
 }