unix/mpthreadport: Fix crash when thread stack size <= 8k.
unix/mpthreadport: Fix crash when thread stack size <= 8k.

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: da39a3ee5e6b4b0d3255bfef95601890afd80709@dpgeorge

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython