macOS: Don't inspect dead threadtls during exception handling. (#50871)

Otherwise we may end up with the state of a dead thread, as the system
IDs can alias. This can lead to an early return from the exception
handler, resulting in e.g. safepoint exceptions being actually delivered
to user code.

---------

Co-authored-by: Jameson Nash <vtjnash@gmail.com>
Co-authored-by: Cody Tapscott <topolarity@tapscott.me>

src/signals-mach.c

@@ -279,12 +279,16 @@ kern_return_t catch_mach_exception_raise(
     int nthreads = jl_atomic_load_acquire(&jl_n_threads);
     for (tid = 0; tid < nthreads; tid++) {
         jl_ptls_t _ptls2 = jl_atomic_load_relaxed(&jl_all_tls_states)[tid];
+        if (jl_atomic_load_relaxed(&_ptls2->current_task) == NULL) {
+            // this thread is dead
+            continue;
+        }
         if (pthread_mach_thread_np(_ptls2->system_id) == thread) {
             ptls2 = _ptls2;
             break;
         }
     }
-    if (!ptls2 || ptls2->current_task == NULL) {
+    if (!ptls2) {
         // We don't know about this thread, let the kernel try another handler
         // instead. This shouldn't actually happen since we only register the
         // handler for the threads we know about.