Clear ivar after scheduling for main thread deallocation #trivial (#590)

* Clear ivar after scheduling for main thread deallocation

After scheduling the ivar for main thread deallocation we have clear out the ivar, otherwise we can run into a race condition where the main queue is drained earlier than this node is deallocated and the ivar is still deallocated on a background thread

* First clear and than schedule

Source/ASDisplayNode.mm

@@ -428,8 +428,18 @@ - (void)_scheduleIvarsForMainDeallocation
 
   for (Ivar ivar : ivars) {
     id value = object_getIvar(self, ivar);
+    if (value == nil) {
+      continue;
+    }
+
     if (ASClassRequiresMainThreadDeallocation(object_getClass(value))) {
       as_log_debug(ASMainThreadDeallocationLog(), "%@: Trampolining ivar '%s' value %@ for main deallocation.", self, ivar_getName(ivar), value);
+
+      // Before scheduling the ivar for main thread deallocation we have clear out the ivar, otherwise we can run
+      // into a race condition where the main queue is drained earlier than this node is deallocated and the ivar
+      // is still deallocated on a background thread
+      object_setIvar(self, ivar, nil);
+
       ASPerformMainThreadDeallocation(value);
     } else {
       as_log_debug(ASMainThreadDeallocationLog(), "%@: Not trampolining ivar '%s' value %@.", self, ivar_getName(ivar), value);