- fixed crash on closing fatal error window

Framebuffer was accessed after its destruction

src/d_main.cpp

@@ -2880,7 +2880,7 @@ void D_Cleanup()
 	M_SaveDefaults(NULL);			// save config before the restart
 	
 	// delete all data that cannot be left until reinitialization
-	screen->CleanForRestart();
+	if (screen) screen->CleanForRestart();
 	V_ClearFonts();					// must clear global font pointers
 	ColorSets.Clear();
 	PainFlashes.Clear();