avformat/matroskaenc: Avoid seek when writing Cues at the front

When the Cues are written in front of the Cluster, the muxer would seek
to the beginning (to where the Cues ought to be written) and write the
Cues; afterwards it would seek back to the end of the file only to seek
to the beginning once again to update several elements there. This
commit removes the seek to the end.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/matroskaenc.c

@@ -2456,7 +2456,6 @@ static int mkv_write_trailer(AVFormatContext *s)
 {
     MatroskaMuxContext *mkv = s->priv_data;
     AVIOContext *pb = s->pb;
-    int64_t currentpos;
     int ret;
 
     // check if we have an audio packet cached
@@ -2479,11 +2478,14 @@ static int mkv_write_trailer(AVFormatContext *s)
 
 
     if ((pb->seekable & AVIO_SEEKABLE_NORMAL) && !mkv->is_live) {
-        int64_t ret64;
+        int64_t endpos, ret64;
+
+        endpos = avio_tell(pb);
 
         if (mkv->cues.num_entries) {
             AVIOContext *cues;
             uint64_t size;
+            int64_t cuespos = endpos;
             int length_size = 0;
 
             ret = start_ebml_master_crc32(&cues, mkv);
@@ -2510,7 +2512,7 @@ static int mkv_write_trailer(AVFormatContext *s)
                     ffio_free_dyn_buf(&cues);
                     goto after_cues;
                 } else {
-                    currentpos = avio_tell(pb);
+                    cuespos = mkv->cues_pos;
                     if ((ret64 = avio_seek(pb, mkv->cues_pos, SEEK_SET)) < 0) {
                         ffio_free_dyn_buf(&cues);
                         return ret64;
@@ -2526,18 +2528,16 @@ static int mkv_write_trailer(AVFormatContext *s)
                     }
                 }
             }
-            mkv_add_seekhead_entry(mkv, MATROSKA_ID_CUES, avio_tell(pb));
+            mkv_add_seekhead_entry(mkv, MATROSKA_ID_CUES, cuespos);
             end_ebml_master_crc32(pb, &cues, mkv, MATROSKA_ID_CUES, length_size);
             if (mkv->reserve_cues_space) {
                 if (size < mkv->reserve_cues_space)
                     put_ebml_void(pb, mkv->reserve_cues_space - size);
-                avio_seek(pb, currentpos, SEEK_SET);
-            }
+            } else
+                endpos = avio_tell(pb);
         }
 
     after_cues:
-        currentpos = avio_tell(pb);
-
         ret = mkv_write_seekhead(pb, mkv, 1, mkv->info_pos);
         if (ret < 0)
             return ret;
@@ -2583,7 +2583,7 @@ static int mkv_write_trailer(AVFormatContext *s)
             end_ebml_master_crc32(pb, &mkv->tags_bc, mkv, MATROSKA_ID_TAGS, 0);
         }
 
-        avio_seek(pb, currentpos, SEEK_SET);
+        avio_seek(pb, endpos, SEEK_SET);
     }
 
     if (!mkv->is_live) {