WinRT: fixed crash when trying, and failing, to load the opengles2 renderer

The crash would occur when a WinRT app explicitly tried to create an
SDL_Renderer using the "opengles2" renderer (via SDL_HINT_RENDER_DRIVER), but
OpenGL ES 2 / ANGLE .dlls weren't packaged in the app.

src/video/winrt/SDL_winrtvideo.cpp

@@ -414,6 +414,7 @@ WINRT_DestroyWindow(_THIS, SDL_Window * window)
         // Delete the internal window data:
         delete data;
         data = NULL;
+        window->driverdata = NULL;
     }
 }