check for emptiness in some more places

these containers can be empty escpecially if one wants to go for a trip
around ncmpcpp screens while connection to mpd is lost. this is highly
unlikely to happen, but should be secured anyway.

src/browser.cpp

@@ -254,6 +254,8 @@ void Browser::ReverseSelection()
 
 void Browser::GetSelectedSongs(MPD::SongList &v)
 {
+	if (w->Empty())
+		return;
 	std::vector<size_t> selected;
 	w->GetSelected(selected);
 	if (selected.empty())

src/playlist.cpp

@@ -280,6 +280,8 @@ MPD::Song *Playlist::CurrentSong()
 
 void Playlist::GetSelectedSongs(MPD::SongList &v)
 {
+	if (myPlaylist->Items->Empty())
+		return;
 	std::vector<size_t> selected;
 	Items->GetSelected(selected);
 	if (selected.empty())

src/search_engine.cpp

@@ -233,6 +233,8 @@ MPD::Song *SearchEngine::CurrentSong()
 
 void SearchEngine::GetSelectedSongs(MPD::SongList &v)
 {
+	if (w->Empty())
+		return;
 	std::vector<size_t> selected;
 	w->GetSelected(selected);
 	if (selected.empty() && w->Choice() >= StaticOptions)

src/tag_editor.cpp

@@ -284,6 +284,8 @@ void TagEditor::Update()
 		Tags->Reset();
 		MPD::SongList list;
 		if (Config.albums_in_tag_editor)
+		{
+			if (!Albums->Empty())
 			{
 				Mpd.StartSearch(1);
 				Mpd.AddSearch(MPD_TAG_ALBUM, Albums->Current().second);
@@ -295,6 +297,7 @@ void TagEditor::Update()
 					Tags->AddOption(**it);
 				}
 			}
+		}
 		else
 		{
 			Mpd.GetSongs(Dirs->Current().second, list);