stackwalk: fix jl_thread_suspend_and_get_state race (#56047)

There was a missing re-assignment of old = -1; at the end of that loop
which means in the ABA case, we accidentally actually acquire the lock
on the thread despite not actually having stopped the thread; or in the
counter-case, we try to run through this logic with old==-1 on the next
iteration, and that isn't valid either (jl_thread_suspend_and_get_state
should return failure and the loop will abort too early).

Fix #56046

src/stackwalk.c

@@ -1196,8 +1196,8 @@ JL_DLLEXPORT size_t jl_record_backtrace(jl_task_t *t, jl_bt_element_t *bt_data,
     }
     bt_context_t *context = NULL;
     bt_context_t c;
-    int16_t old = -1;
-    while (!jl_atomic_cmpswap(&t->tid, &old, ptls->tid) && old != ptls->tid) {
+    int16_t old;
+    for (old = -1; !jl_atomic_cmpswap(&t->tid, &old, ptls->tid) && old != ptls->tid; old = -1) {
         int lockret = jl_lock_stackwalk();
         // if this task is already running somewhere, we need to stop the thread it is running on and query its state
         if (!jl_thread_suspend_and_get_state(old, 1, &c)) {