hle: kernel: KThread: Decrease DummyThread priority to ensure it is never scheduled.

src/core/hle/kernel/k_scheduler.cpp

@@ -741,6 +741,7 @@ void KScheduler::ScheduleImpl() {
 
     // We never want to schedule a dummy thread, as these are only used by host threads for locking.
     if (next_thread->GetThreadType() == ThreadType::Dummy) {
+        ASSERT_MSG(false, "Dummy threads should never be scheduled!");
         next_thread = idle_thread;
     }