Don't call SDL_GetWindowDisplayIndex() with window == NULL

I got an SDL assertion because of this.
The problem was that PrintDisplayModes() was called at startup before a
window was created.

src/client/vid/glimp_sdl.c

@@ -263,10 +263,11 @@ InitDisplayIndices()
 static void
 PrintDisplayModes(void)
 {
-	int curdisplay = SDL_GetWindowDisplayIndex(window);
+	int curdisplay = window ? SDL_GetWindowDisplayIndex(window) : 0;
 
 	// On X11 (at least for me)
 	// curdisplay is always -1.
+	// DG: probably because window was NULL?
 	if (curdisplay < 0) {
 		curdisplay = 0;
 	}