don't lose mark when pp_reverse extends stack

don't lose mark when pp_reverse extends stack

Nullary reverse needs to extend the stack to push its result scalar.
It was actually extending the stack, but doing so invalidated MARK,
which it relied upon to place the stack pointer afterwards.  Upon stack
reallocation it was therefore leaving the stack pointer pointing to the
freed stack memory.  Reformulate stack manipulation to not rely on MARK
after extending.  Fixes [perl #132544].