[KMSDRM] Fix segmentation fault

Deference the windata pointer *after* checking that it's non-NULL.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

src/video/kmsdrm/SDL_kmsdrmvideo.c

@@ -1071,7 +1071,7 @@ KMSDRM_DestroyWindow(_THIS, SDL_Window *window)
 {
     SDL_WindowData *windata = (SDL_WindowData *) window->driverdata;
     SDL_DisplayData *dispdata = (SDL_DisplayData *) SDL_GetDisplayForWindow(window)->driverdata;
-    SDL_VideoData *viddata = windata->viddata;
+    SDL_VideoData *viddata;
     SDL_bool is_vulkan = window->flags & SDL_WINDOW_VULKAN; /* Is this a VK window? */
     unsigned int i, j;
 
@@ -1079,6 +1079,8 @@ KMSDRM_DestroyWindow(_THIS, SDL_Window *window)
         return;
     }
 
+    viddata = windata->viddata;
+
     if ( !is_vulkan && viddata->gbm_init) {
 
         /* Destroy cursor GBM BO of the display of this window. */