refactor: remove unnecessary intermediate buffer for ffmpeg image extraction

Signed-off-by: Deluan <deluan@navidrome.org>

core/artwork/sources.go

@@ -141,13 +141,7 @@ func fromFFmpegTag(ctx context.Context, ffmpeg ffmpeg.FFmpeg, path string) sourc
 		if err != nil {
 			return nil, "", err
 		}
-		defer r.Close()
-		buf := new(bytes.Buffer)
-		_, err = io.Copy(buf, r)
-		if err != nil {
-			return nil, "", err
-		}
-		return io.NopCloser(buf), path, nil
+		return r, path, nil
 	}
 }