[ASDisplayNode] Ensure that nil can never be returned from -measureWithSizeRange: (#2014)

* [ASDisplayNode] Ensure that nil can never be returned from -measureWithSizeRange:

This can happen in rare cases when multiple relayouts occur while a transition is being measured.

* [ASDisplayNode] Use ternary operator style for nil check.

Author: appleguy

AsyncDisplayKit/ASDisplayNode.mm

@@ -642,7 +642,8 @@ - (ASLayout *)measureWithSizeRange:(ASSizeRange)constrainedSize
 {
   ASDN::MutexLocker l(__instanceLock__);
   if (! [self shouldMeasureWithSizeRange:constrainedSize]) {
-    return _calculatedLayout;
+    ASDisplayNodeAssertNotNil(_calculatedLayout, @"-[ASDisplayNode measureWithSizeRange:] _layout should not be nil! %@", self);
+    return _calculatedLayout ? : [ASLayout layoutWithLayoutableObject:self constrainedSizeRange:constrainedSize size:CGSizeZero];
   }
 
   [self cancelLayoutTransitionsInProgress];
@@ -659,6 +660,7 @@ - (ASLayout *)measureWithSizeRange:(ASSizeRange)constrainedSize
     [self _completePendingLayoutTransition];
   }
 
+  ASDisplayNodeAssertNotNil(newLayout, @"-[ASDisplayNode measureWithSizeRange:] newLayout should not be nil! %@", self);
   return newLayout;
 }
 
@@ -2071,6 +2073,8 @@ - (ASLayout *)calculateLayoutThatFits:(ASSizeRange)constrainedSize
 
     layoutSpec.isMutable = NO;
     ASLayout *layout = [layoutSpec measureWithSizeRange:constrainedSize];
+    ASDisplayNodeAssertNotNil(layout, @"[ASLayoutSpec measureWithSizeRange:] should never return nil! %@, %@", self, layoutSpec);
+      
     // Make sure layoutableObject of the root layout is `self`, so that the flattened layout will be structurally correct.
     BOOL isFinalLayoutable = (layout.layoutableObject != self);
     if (isFinalLayoutable) {

AsyncDisplayKit/Private/ASStackUnpositionedLayout.mm

@@ -28,7 +28,9 @@
   // stretched children will have a cross dimension of at least crossMin
   const CGFloat childCrossMin = alignItems == ASStackLayoutAlignItemsStretch ? crossMin : 0;
   const ASSizeRange childSizeRange = directionSizeRange(style.direction, stackMin, stackMax, childCrossMin, crossMax);
-  return [child measureWithSizeRange:childSizeRange];
+  ASLayout *layout = [child measureWithSizeRange:childSizeRange];
+  ASDisplayNodeCAssertNotNil(layout, @"ASLayout returned from measureWithSizeRange: must not be nil: %@", child);
+  return layout ? : [ASLayout layoutWithLayoutableObject:child constrainedSizeRange:childSizeRange size:CGSizeZero];
 }
 
 /**