op.c: S_lvref(): handle all kids on OP_NULL

op.c: S_lvref(): handle all kids on OP_NULL

For an OP_NULL, his function formerly recursed into *all* its kids
if was an ex-list, otherwise only the first one.

To simplify making this function non-recursive, make it so that it
unconditionally recurses into all the kids.

However for now, also add an assertion that a non ex-list OP_NULL
will only have one child at most. If we find some code which violates
this, then we can nmake a more informed decision as to whether
non ex-list OP_NULL's should have all, or only their first child
examined.