[ASImageNode] Fix a threading issue which can cause a display comple…

…tion block to never be executed (TextureGroup#1148)

- Clear _displayCompletionBlock while we still have the node's instance lock. Because it may not be the same block by the time the lock is reacquired. In other words, it can happen that another thread sets a new display block after this thread releases the lock but before it reacquires it. And we don't want to clear out the new block.
- Reduce a lock/unlock pair which should help perf a tiny bit.

Source/ASImageNode.mm

@@ -533,8 +533,15 @@ - (void)displayDidFinish
   [super displayDidFinish];
 
   __instanceLock__.lock();
-    void (^displayCompletionBlock)(BOOL canceled) = _displayCompletionBlock;
     UIImage *image = _image;
+    void (^displayCompletionBlock)(BOOL canceled) = _displayCompletionBlock;
+    BOOL shouldPerformDisplayCompletionBlock = (image && displayCompletionBlock);
+
+    // Clear the ivar now. The block is retained and will be executed shortly.
+    if (shouldPerformDisplayCompletionBlock) {
+      _displayCompletionBlock = nil;
+    }
+
     BOOL hasDebugLabel = (_debugLabelNode != nil);
   __instanceLock__.unlock();
 
@@ -556,13 +563,8 @@ - (void)displayDidFinish
   }
 
   // If we've got a block to perform after displaying, do it.
-  if (image && displayCompletionBlock) {
-
+  if (shouldPerformDisplayCompletionBlock) {
     displayCompletionBlock(NO);
-
-    __instanceLock__.lock();
-      _displayCompletionBlock = nil;
-    __instanceLock__.unlock();
   }
 }