Move all rebinding checks into one function to make sure they stay
in sync. Normalize return value to be NULL for all rebinding
failures, instead of returning an improperly bound closure in some
cases.
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());
