Fix a crash if the gpu doesn't load.

E.g. for invalid files with multithreading enabled.

Core/System.cpp

@@ -260,8 +260,10 @@ void CPU_RunLoop() {
 	}
 
 	// Let's make sure the gpu has already cleaned up before we start freeing memory.
-	gpu->FinishEventLoop();
-	gpu->SyncThread(true);
+	if (gpu) {
+		gpu->FinishEventLoop();
+		gpu->SyncThread(true);
+	}
 
 	CPU_Shutdown();
 	CPU_SetState(CPU_THREAD_NOT_RUNNING);