avformat/matroskaenc: Improve checks for updating Tags

When updating the Tags at the end, the Matroska muxer would twice check
for whether (!mkv->is_live) is true, despite this code being only executed
if it is. Furthermore, a loop iterates over all the streams even when
there is no Tags element to update at all, because the check for whether
there are Tags is only performed later. This commit fixes this.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/matroskaenc.c

@@ -2545,7 +2545,7 @@ static int mkv_write_trailer(AVFormatContext *s)
         end_ebml_master_crc32(pb, &mkv->tracks_bc, mkv, MATROSKA_ID_TRACKS, 0, 0);
 
         // update stream durations
-        if (!mkv->is_live) {
+        if (mkv->tags_bc) {
             int i;
             for (i = 0; i < s->nb_streams; ++i) {
                 AVStream *st = s->streams[i];
@@ -2567,8 +2567,7 @@ static int mkv_write_trailer(AVFormatContext *s)
                     put_ebml_binary(mkv->tags_bc, MATROSKA_ID_TAGSTRING, duration_string, 20);
                 }
             }
-        }
-        if (mkv->tags_bc && !mkv->is_live) {
+
             avio_seek(pb, mkv->tags_pos, SEEK_SET);
             end_ebml_master_crc32(pb, &mkv->tags_bc, mkv, MATROSKA_ID_TAGS, 0, 0);
         }