[FIX] component: protect against user code executing in critical section

Canceling a fiber may cause user code to be run, which means that some
new renderings could be scheduled, but this could interfere with the
current renderings!

src/component/component_node.ts

@@ -79,10 +79,8 @@ export function component<P extends object>(
   let node: any = ctx.children[key];
   let isDynamic = typeof name !== "string";
 
-  if (node) {
-    if (node.status === STATUS.DESTROYED) {
-      node = undefined;
-    }
+  if (node && node.status === STATUS.DESTROYED) {
+    node = undefined;
   }
   if (isDynamic && node && node.component.constructor !== name) {
     node = undefined;

src/component/fibers.ts

@@ -16,7 +16,12 @@ export function makeRootFiber(node: ComponentNode): Fiber {
   let current = node.fiber;
   if (current) {
     let root = current.root!;
+    // lock root fiber because canceling children fibers may destroy components,
+    // which means any arbitrary code can be run in onWillDestroy, which may
+    // trigger new renderings
+    root.locked = true;
     root.setCounter(root.counter + 1 - cancelFibers(current.children));
+    root.locked = false;
     current.children = [];
     current.childrenMap = {};
     current.bdom = null;