thread: fix inconsistent return values

- SDL_CreateMutex returns NULL when the creation fails (ngage)
- SDL_SemValue returns 0 when the semaphore is NULL (n3ds)

src/thread/ngage/SDL_sysmutex.cpp

@@ -47,6 +47,7 @@ SDL_CreateMutex(void)
     TInt status = CreateUnique(NewMutex, &rmutex, NULL);
     if (status != KErrNone) {
         SDL_SetError("Couldn't create mutex.");
+        return NULL;
     }
     SDL_mutex* mutex = new /*(ELeave)*/ SDL_mutex;
     mutex->handle = rmutex.Handle();