pp_return: optimise a couple of conditions

pp_return: optimise a couple of conditions

Change:

    if (cxix < 0) {
        A; return;
    }
    if (cxix < cxstack_ix)
        B;

to

    if (cxix < cxstack_ix) {
        if (cxix < 0) {
            A; return;
        }
        B;
    }

This is functionally the same, since cxstack_ix is always positive at
this point, and makes for a quicker code path (one less test and branch)
in the reasonably common case of a return from a sub which doesn't
have any extra nested contexts to pop.