Playlist: More defensive proramming for when trying to play an empty playlist.

src/playlist.cpp

@@ -310,8 +310,13 @@ void Playlist::reset() {
 	_started = false;
 }
 
-String Playlist::get_current_file() {
-	return _files[_current_track].filename;
+bool Playlist::get_current_file(String* dst) {
+	if (_current_track < _files.size()) {
+		return false;
+	} else {
+		dst->concat(_files[_current_track].filename);
+		return true;
+	}
 }
 
 uint32_t Playlist::get_position() {