py/modthread: Initialise nlr_jump_callback_top on threads.

The main thread gets this because the thread state is in bss, but
subsequent threads need this field to be initialised.

Also added a note to mpstate.h to help avoid missing this in the future.

Fixes issue #12695.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

py/modthread.c

@@ -174,6 +174,7 @@ STATIC void *thread_entry(void *args_in) {
     // The GC starts off unlocked on this thread.
     ts.gc_lock_depth = 0;
 
+    ts.nlr_jump_callback_top = NULL;
     ts.mp_pending_exception = MP_OBJ_NULL;
 
     // set locals and globals from the calling context