MSVC: Fix SDL binary assertion failure on start.

sdlReadDesktopVideoMode() was apparently being called when the window
pointer is NULL.

Wrap the code in an `if (window) { ... }`.

SDL binary now launches correctly.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

src/sdl/SDL.cpp

@@ -763,11 +763,13 @@ void sdlReadBattery()
 
 void sdlReadDesktopVideoMode()
 {
+    if (window) {
         SDL_DisplayMode dm;
         SDL_GetDesktopDisplayMode(SDL_GetWindowDisplayIndex(window), &dm);
         desktopWidth = dm.w;
         desktopHeight = dm.h;
     }
+}
 
 static void sdlResizeVideo()
 {