[ASLayout] Add additional validation step to intercept incorrect values in production and safely zero them out.

These conditions already had assertions, but at runtime, an insufficiently tested and incorrect ASLayoutSpec could
generate values that cause UIKit to enter an infinite loop (e.g. inside of UICollectionView layout validation).

AsyncDisplayKit/Layout/ASLayout.h

@@ -8,6 +8,8 @@
  *
  */
 
+#pragma once
+
 #import <UIKit/UIKit.h>
 #import <AsyncDisplayKit/ASAssert.h>
 #import <AsyncDisplayKit/ASLayoutable.h>