Tighten asserts on ParallelWorkerNumber a little bit

Hi,

While hacking on something I happened to notice a couple asserts on
ParallelWorkerNumber when collecting shared instrumentation:

Assert(ParallelWorkerNumber <= node->shared_info->num_workers);

This is not quire right, because num_workers is used to size arrays
indexed by ParallelWorkerNumber. And the comment in parallel.c also
claims (ParallelWorkerNumber < num_workers).

So AFAICS the assert(s) should be

Assert(ParallelWorkerNumber < node->shared_info->num_workers);

I don't think we had issues with this not catching a bug. But it may be
a bit misleading, so worth fixing and (probably) backpatching.

regards

--
Tomas Vondra