ubusd: fix sending remove-object notification

ubusd: fix sending remove-object notification

Patch by Delio Brignoli <brignoli.delio@gmail.com>

Both ubusd_free_object (eventually via ubusd_create_object_event_msg)
and ubus_proto_send_msg_from_blob() use the same message buffer.
So ubusd_handle_remove_object builds the payload which gets (indirectly)
overwritten by the call to ubusd_free_object and then sent again by
ubus_proto_send_msg_from_blob.

Signed-off-by: Felix Fietkau <nbd@nbd.name>