fix segfault that occured if mpd was stopped and window had to be resized

we all love null pointer dereferences, don't we?
some methods from Song class need assertion guards to deal with that.

src/status.cpp

@@ -347,11 +347,9 @@ void NcmpcppStatusChanged(Connection *, StatusChanges changed, void *)
 	{
 		if (myPlaylist->isPlaying())
 		{
-			np = Mpd.GetCurrentSong();
-			
 			if (!Config.execute_on_song_change.empty())
 				system(Config.execute_on_song_change.c_str());
-			if (Mpd.isPlaying())
+			if (Mpd.isPlaying() && !(np = Mpd.GetCurrentSong()).Empty())
 				WindowTitle(utf_to_locale_cpy(np.toString(Config.song_window_title_format)));
 			if (Config.autocenter_mode && !myPlaylist->Items->isFiltered())
 				myPlaylist->Items->Highlight(myPlaylist->NowPlaying);
@@ -373,11 +371,8 @@ void NcmpcppStatusChanged(Connection *, StatusChanges changed, void *)
 	if (((now > past || changed.SongID) && Mpd.isPlaying()) || RedrawStatusbar)
 	{
 		time(&past);
-		if (np.Empty())
-		{
-			np = Mpd.GetCurrentSong();
+		if (np.Empty() && !(np = Mpd.GetCurrentSong()).Empty())
 			WindowTitle(utf_to_locale_cpy(np.toString(Config.song_window_title_format)));
-		}
 		if (!np.Empty() && Mpd.isPlaying())
 		{
 			changed.ElapsedTime = 1;