Changed SDL_ClearError() to always return 0.

This will provide a quick and easy way of clearing the error when a function succeeds, if we want to do that in a more widespread way.

For now we guarantee that SDL_Init() will never have an error set when it returns successfully.

Fixes https://github.com/libsdl-org/SDL/issues/8421

src/SDL.c

@@ -400,7 +400,7 @@ int SDL_InitSubSystem(Uint32 flags)
 
     (void)flags_initialized; /* make static analysis happy, since this only gets used in error cases. */
 
-    return 0;
+    return SDL_ClearError();
 
 quit_and_error:
     SDL_QuitSubSystem(flags_initialized);