Prevent capture on non running node

Fix https://github.com/GNS3/gns3-gui/issues/1656

gns3server/controller/compute.py

@@ -410,7 +410,7 @@ class Compute:
                 self._memory_usage_percent = event["memory_usage_percent"]
                 self._controller.notification.emit("compute.updated", self.__json__())
             else:
-                self._controller.notification.dispatch(action, event, compute_id=self.id)
+                yield from self._controller.notification.dispatch(action, event, compute_id=self.id)
         if self._ws:
             yield from self._ws.close()