Scheduler: Fix HLE Threads on guard

src/core/hle/kernel/scheduler.cpp

@@ -689,11 +689,13 @@ void Scheduler::SwitchToCurrent() {
         current_thread = selected_thread;
         guard.unlock();
         while (!is_context_switch_pending) {
+            if (current_thread != nullptr) {
                 current_thread->context_guard.lock();
                 if (current_thread->GetSchedulingStatus() != ThreadSchedStatus::Runnable) {
                     current_thread->context_guard.unlock();
                     break;
                 }
+            }
             std::shared_ptr<Common::Fiber> next_context;
             if (current_thread != nullptr) {
                 next_context = current_thread->GetHostContext();