Produce error if unable to create WS listener.

src/mosquitto.c

@@ -300,6 +300,10 @@ int main(int argc, char *argv[])
 		}else if(config.listeners[i].protocol == mp_websockets){
 #ifdef WITH_WEBSOCKETS
 			config.listeners[i].ws_context = mosq_websockets_init(&config.listeners[i]);
+			if(!config.listeners[i].ws_context){
+				_mosquitto_log_printf(NULL, MOSQ_LOG_ERR, "Error: Unable to create websockets listener on port %d.", config.listeners[i].port);
+				return 1;
+			}
 #endif
 		}
 	}