Improve set-constant.js scriptlet

The scriptlet will now still try to trap a specific
property if a segment of the chain is not undefined
while yet not an object either.

For example, this now allows to set a value on
`document.body.onselectstart` when `document.body` has
not been instantiated yet by the browser parser,
whereas this would previously fail because
`document.body` would be `null` while the scriptlet
was testing against `undefined`.

assets/resources/resources.txt

@@ -1696,7 +1696,7 @@ set-constant.js application/javascript
 		var prop = chain.slice(0, pos),
 			v = owner[prop];
 		chain = chain.slice(pos + 1); 
-		if ( v !== undefined ) {
+		if ( v instanceof Object ) {
 			makeProxy(v, chain);
 			return;
 		}