[FIX] component: concurrent rendering issue

Resolved rendering with cancelled fiber for (not yet) destroyed
component -> Cannot read property 'sel' of null

Closes #421

src/component/component.ts

@@ -438,6 +438,9 @@ export class Component<T extends Env, Props extends {}> {
     }
     __owl__.isDestroyed = true;
     delete __owl__.vnode;
+    if (__owl__.currentFiber) {
+      __owl__.currentFiber.isCancelled = true;
+    }
   }
 
   __callMounted() {