Update 2018-05-23-react-v-16-4.md example fix (#1423)

Minor "fix" to the suggested fix in the blog post to default back to empty object if `state.prevProps` isn't yet set. This will help anyone copying the example as-is who has stumbled across this blog post.

content/blog/2018-05-23-react-v-16-4.md

@@ -62,7 +62,7 @@ One possible way to fix this is to compare the incoming value to the previous va
 
 ```js
 static getDerivedStateFromProps(props, state) {
-  const prevProps = state.prevProps;
+  const prevProps = state.prevProps || {};
   // Compare the incoming prop to previous prop
   const controlledValue =
     prevProps.value !== props.value