thread: fix inconsistent return values

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

(cherry picked from commit 6875e1c262)

src/thread/n3ds/SDL_syssem.c

@@ -114,7 +114,8 @@ Uint32
 SDL_SemValue(SDL_sem *sem)
 {
     if (sem == NULL) {
-        return SDL_InvalidParamError("sem");
+        SDL_InvalidParamError("sem");
+        return 0;
     }
     return sem->semaphore.current_count;
 }