sd-bus: when an event loop terminates, explicitly close the bus

This makes sure we actually release the bus and all the messages it
references.

src/libsystemd/sd-bus/sd-bus.c

@@ -2940,6 +2940,7 @@ static int quit_callback(sd_event_source *event, void *userdata) {
         assert(event);
 
         sd_bus_flush(bus);
+        sd_bus_close(bus);
 
         return 1;
 }