Fix visibility assert on iOS 7

It appears that didMoveToViewController: can be called with nil,
yet self.parentViewController will *not* be nil. This can result
in calling parent view controller's visibility depth. Instead of
asserting, these methods should return NSNotFound which also happens
to be a really large number, effectively infinite depth.

AsyncDisplayKit/ASNavigationController.m

@@ -40,7 +40,10 @@ ASVisibilityDepthImplementation;
 - (NSInteger)visibilityDepthOfChildViewController:(UIViewController *)childViewController
 {
   NSUInteger viewControllerIndex = [self.viewControllers indexOfObject:childViewController];
-  NSAssert(viewControllerIndex != NSNotFound, @"childViewController is not in the navigation stack.");
+  if (viewControllerIndex == NSNotFound) {
+    //If childViewController is not actually a child, return NSNotFound which is also a really large number.
+    return NSNotFound;
+  }
   
   if (viewControllerIndex == self.viewControllers.count - 1) {
     //view controller is at the top, just return our own visibility depth.

AsyncDisplayKit/ASTabBarController.m

@@ -39,6 +39,12 @@ ASVisibilityDepthImplementation;
 
 - (NSInteger)visibilityDepthOfChildViewController:(UIViewController *)childViewController
 {
+  NSUInteger viewControllerIndex = [self.viewControllers indexOfObject:childViewController];
+  if (viewControllerIndex == NSNotFound) {
+    //If childViewController is not actually a child, return NSNotFound which is also a really large number.
+    return NSNotFound;
+  }
+  
   if (self.selectedViewController == childViewController) {
     return [self visibilityDepth];
   }