lyrics: fix segfault if song didn't have proper tags set

src/lyrics.cpp

@@ -110,18 +110,20 @@ void Lyrics::SwitchTo()
 #	endif // HAVE_CURL_CURL_H
 	
 	if (const MPD::Song *s = myScreen->CurrentSong())
+	{
+		if (!s->GetArtist().empty() && !s->GetTitle().empty())
 		{
 			myOldScreen = myScreen;
 			myScreen = this;
 			
-		if (!s->GetArtist().empty() && !s->GetTitle().empty())
-		{
 			itsSong = *s;
 			Load();
-		}
 			
 			Global::RedrawHeader = 1;
 		}
+		else
+			ShowMessage("Song must have both artist and title tag set!");
+	}
 }
 
 std::basic_string<my_char_t> Lyrics::Title()