make it possible to pop all items in a scope chain and then push to it

A lot of the JSC::ExecState functions rely on scopeChain() not
being 0. This means we shouldn't pop the scope chain if doing so
would make it empty; otherwise when you call e.g. pushScope() or
evaluate(), you will crash. So instead of popping the chain
completely, we now set the sole scope chain item's object pointer to
0 and add appropriate checks elsewhere.

A second issue, not solved in this commit, is that JSC expects
the Global Object to always be the last item in every scope
chain. If it's not, you will crash.

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