The stack size adjustment for detecting stack overflow in threads was not
taking into account that the requested stack size could be <= 8k, in which
case the subtraction would overflow. This is fixed in this commit by
ensuring that the adjustment can't be more than the available size.
This fixes the test tests/thread/thread_stacksize1.py which sometimes
crashes with a segmentation fault because of an uncaught NLR jump, which is
a "maximum recursion depth exceeded" exception.
Suggested-by: @dpgeorge
5e6cee07ab