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: @dpgeorge

ports/unix/mpthreadport.c

@@ -221,7 +221,11 @@ void mp_thread_create(void *(*entry)(void *), void *arg, size_t *stack_size) {
 
     // adjust stack_size to provide room to recover from hitting the limit
     // this value seems to be about right for both 32-bit and 64-bit builds
-    *stack_size -= 8192;
+    if (*stack_size >= 2 * 8192) {
+        *stack_size -= 8192;
+    } else {
+        *stack_size /= 2;
+    }
 
     // add thread to linked list of all threads
     thread_t *th = malloc(sizeof(thread_t));