Kernel Scheduler: Make sure the global scheduler shutdowns correctly.

src/core/hle/kernel/scheduler.h

@@ -147,6 +147,8 @@ public:
         return reselection_pending.load();
     }
 
+    void Shutdown();
+
 private:
     bool AskForReselectionOrMarkRedundant(Thread* current_thread, Thread* winner);
 
@@ -189,6 +191,11 @@ public:
         return context_switch_pending;
     }
 
+    void Shutdown() {
+        current_thread = nullptr;
+        selected_thread = nullptr;
+    }
+
 private:
     friend class GlobalScheduler;
     /**