Properly test for the lack of a node measure func in YogaKit (#915)

Summary:
The test was broken and caused a few crashes on my project.
Pull Request resolved: https://github.com/facebook/yoga/pull/915

Differential Revision: D16701613

Pulled By: davidaurelio

fbshipit-source-id: 9ab5c43bb74e77593bc2426a249750a7ee8f4034

YogaKit/Source/YGLayout.m

@@ -180,7 +180,7 @@ static YGConfigRef globalConfig;
   // the measure function. Since we already know that this is a leaf,
   // this *should* be fine. Forgive me Hack Gods.
   const YGNodeRef node = self.node;
-  if (YGNodeHasMeasureFunc(node)) {
+  if (!YGNodeHasMeasureFunc(node)) {
     YGNodeSetMeasureFunc(node, YGMeasureView);
   }