Add NULL checks and asserts to graphic options

The game dereferences graphics.screenbuffer without checking it first...
it's unlikely to happen, but the least we can to do be safe is to add a
check and assert here.

desktop_version/src/Input.cpp

@@ -445,6 +445,12 @@ static void menuactionpress(void)
         map.nexttowercolour();
         break;
     case Menu::graphicoptions:
+        if (graphics.screenbuffer == NULL)
+        {
+            SDL_assert(0 && "Screenbuffer is NULL!");
+            break;
+        }
+
         switch (game.currentmenuoption)
         {
         case 0: