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_error.c

@@ -78,13 +78,14 @@ const char *SDL_GetError(void)
     }
 }
 
-void SDL_ClearError(void)
+int SDL_ClearError(void)
 {
     SDL_error *error = SDL_GetErrBuf(SDL_FALSE);
 
     if (error) {
         error->error = SDL_ErrorCodeNone;
     }
+    return 0;
 }
 
 /* Very common errors go here */