[FIX] component: resolve a subtle concurrency issue

Asynchronous rendering is subtle. Before this commit, it was possible
(though not easy) to get into a situation where the Owl rendering
pipeline decremented twice the counter of a fiber that indicates that
its work is complete.

The situation occurs in the `render` method, where we create a fiber,
wait for a micro tick before starting the actual rendering (the goal was
to batch all changes coming from a single call stack). However, in this
microtask tick, it was possible that the fiber was cancelled, and we did
not have a check for that.

src/component/component.ts

@@ -373,6 +373,9 @@ export class Component<Props extends {} = any, T extends Env = Env> {
     const fiber = new Fiber(null, this, force, null);
     Promise.resolve().then(() => {
       if (__owl__.isMounted || !isMounted) {
+        if (fiber.isCompleted) {
+          return;
+        }
         // we are mounted (__owl__.isMounted), or if we are currently being
         // mounted (!isMounted), so we call __render
         this.__render(fiber);