Allow random $this on non-internal Closures again

Allow random $this on non-internal Closures again
As it turns out, there is actually no reason to prevent this, it even was a bigger BC break than expected...

Also fixes a memory leak (the Closure leaks) when calling internal functions via Closure by moving it out of leave helper onto caller side for TOP_CODE:

$z = new SplStack; $z->push(20);
$x = (new ReflectionMethod("SplStack", "pop"))->getClosure($z);
var_dump($x());