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

@@ -285,14 +285,17 @@ void TagEditor::Update()
 		MPD::SongList list;
 		if (Config.albums_in_tag_editor)
 		{
-			Mpd.StartSearch(1);
-			Mpd.AddSearch(MPD_TAG_ALBUM, Albums->Current().second);
-			Mpd.CommitSearch(list);
-			sort(list.begin(), list.end(), CaseInsensitiveSorting());
-			for (MPD::SongList::iterator it = list.begin(); it != list.end(); ++it)
+			if (!Albums->Empty())
 			{
-				(*it)->Localize();
-				Tags->AddOption(**it);
+				Mpd.StartSearch(1);
+				Mpd.AddSearch(MPD_TAG_ALBUM, Albums->Current().second);
+				Mpd.CommitSearch(list);
+				sort(list.begin(), list.end(), CaseInsensitiveSorting());
+				for (MPD::SongList::iterator it = list.begin(); it != list.end(); ++it)
+				{
+					(*it)->Localize();
+					Tags->AddOption(**it);
+				}
 			}
 		}
 		else