X11TK: fix late null-check causing segfault

The code is using `controls.window` before checking if it isn't null.

I found this bug by accident when I tried to run `SDL_ShowSimpleMessageBox`. 

It first tried using Wayland with `zenity`, but since I don't have `zenity`, it fallbacked to `X11_ShowMessageBoxImpl`.

For some reason it couldn't create a window, maybe something related to XWayland, so `controls.window` was `NULL`.

src/video/x11/SDL_x11messagebox.c

@@ -250,11 +250,11 @@ static bool X11_ShowMessageBoxImpl(const SDL_MessageBoxData *messageboxdata, int
 #else 
     controls.window = X11Toolkit_CreateWindowStruct(parent_window, NULL, SDL_TOOLKIT_WINDOW_MODE_X11_DIALOG, colorhints, false);
 #endif
-    controls.window->cb_data = &controls;
-    controls.window->cb_on_scale_change = X11_OnMessageBoxScaleChange;
     if (!controls.window) {
         return false;
     }
+    controls.window->cb_data = &controls;
+    controls.window->cb_on_scale_change = X11_OnMessageBoxScaleChange;
 
     /* Create controls */
     controls.buttonID = buttonID;