Improve thread-safety of ASDisplayNode's didExitHierarchy #trivial (#916

)

Since `didExitHierarchy` is called without the node's instance lock being held, the lock must be acquired before accessing `pendingInterfaceState`.

Source/ASDisplayNode.mm

@@ -3016,7 +3016,7 @@ - (void)didExitHierarchy
     return;
   }
 #endif
-  if (ASInterfaceStateIncludesVisible(_pendingInterfaceState)) {
+  if (ASInterfaceStateIncludesVisible(self.pendingInterfaceState)) {
     void(^exitVisibleInterfaceState)(void) = ^{
       // This block intentionally retains self.
       __instanceLock__.lock();