playlist/Plugin: use std::unique_ptr<SongEnumerator>

src/playlist/plugins/ExtM3uPlaylistPlugin.cxx

@@ -59,18 +59,16 @@ public:
 	virtual std::unique_ptr<DetachedSong> NextSong() override;
 };
 
-static SongEnumerator *
+static std::unique_ptr<SongEnumerator>
 extm3u_open_stream(InputStreamPtr &&is)
 {
-	ExtM3uPlaylist *playlist = new ExtM3uPlaylist(std::move(is));
+	auto playlist = std::make_unique<ExtM3uPlaylist>(std::move(is));
 
 	is = playlist->CheckFirstLine();
-	if (is) {
+	if (is)
 		/* no EXTM3U header: fall back to the plain m3u
 		   plugin */
-		delete playlist;
-		return nullptr;
-	}
+		playlist.reset();
 
 	return playlist;
 }