greybus: operation: fix null-deref on operation destroy

Incoming operations are created without a response message. If a
protocol driver fails to send a response, or if the operation were to be
cancelled before it has been fully processed, we get a null-pointer
dereference when the operation is released.
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/staging/greybus/operation.c

@@ -607,7 +607,8 @@ static void _gb_operation_destroy(struct kref *kref)
 	list_del(&operation->links);
 	spin_unlock_irqrestore(&gb_operations_lock, flags);
 
-	gb_operation_message_free(operation->response);
+	if (operation->response)
+		gb_operation_message_free(operation->response);
 	gb_operation_message_free(operation->request);
 
 	kmem_cache_free(gb_operation_cache, operation);