The original code tried to set the stack pointer and call the closure
_in the same stack frame_. This isn't possible, so the actual stack
being used was the 8K stack setup by ROM code. If this stack overflowed,
it could corrupt the ROM function .bss and .data which is why I was
seeing intermittent panic messages. I still don't understand all the
failure cases, but I know that now we are using the correct stack
properly.