Fix potential crash with Eevee render of missing image textures

This NULL check is needed elsewhere, do it here as well.

source/blender/gpu/intern/gpu_draw.c

@@ -880,7 +880,7 @@ GPUTexture *GPU_texture_from_blender(Image *ima, ImageUser *iuser, ImBuf *ibuf,
    * texture with zero bindcode so we don't keep trying. */
   uint bindcode = 0;
   ImageTile *tile = BKE_image_get_tile(ima, 0);
-  if (tile->ok == 0) {
+  if (tile == NULL || tile->ok == 0) {
     *tex = GPU_texture_from_bindcode(textarget, bindcode);
     return *tex;
   }