Nullify scheduler during terminate_atfork_i. (ruby#15354)

Author: samuel-williams-shopify

thread.c

@@ -4979,6 +4979,9 @@ static void
 terminate_atfork_i(rb_thread_t *th, const rb_thread_t *current_th)
 {
     if (th != current_th) {
+        // Clear the scheduler as it is no longer operational:
+        th->scheduler = Qnil;
+
         rb_native_mutex_initialize(&th->interrupt_lock);
         rb_mutex_abandon_keeping_mutexes(th);
         rb_mutex_abandon_locking_mutex(th);