fix before diff hook being called on invalid vnodes

Author: developit

src/pretty.js

@@ -90,11 +90,11 @@ function _renderToStringPretty(
 		return rendered;
 	}
 
-	if (options[DIFF]) options[DIFF](vnode);
-
 	// VNodes have {constructor:undefined} to prevent JSON injection:
 	if (vnode.constructor !== undefined) return '';
 
+	if (options[DIFF]) options[DIFF](vnode);
+
 	let nodeName = vnode.type,
 		props = vnode.props,
 		isComponent = false;