avformat/matroskadec: fix IGNIDX handling

Just because the user requested the seek index to be ignored, we can't
just skip essential headers. At least tags are often located at the end
of the file, and the old code simply ignored the seekhead for all
elements, not just the cue index. Also, it looks like it used the index
even if IGNIDX was set if the cue index was located in the beginning of
the file.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskadec.c

@@ -1464,8 +1464,7 @@ static void matroska_execute_seekhead(MatroskaDemuxContext *matroska)
     int i;
 
     // we should not do any seeking in the streaming case
-    if (!matroska->ctx->pb->seekable ||
-        (matroska->ctx->flags & AVFMT_FLAG_IGNIDX))
+    if (!matroska->ctx->pb->seekable)
         return;
 
     for (i = 0; i < seekhead_list->nb_elem; i++) {
@@ -1500,6 +1499,9 @@ static void matroska_add_index_entries(MatroskaDemuxContext *matroska)
     int index_scale = 1;
     int i, j;
 
+    if (matroska->ctx->flags & AVFMT_FLAG_IGNIDX)
+        return;
+
     index_list = &matroska->index;
     index      = index_list->elem;
     if (index_list->nb_elem &&
@@ -1525,6 +1527,9 @@ static void matroska_add_index_entries(MatroskaDemuxContext *matroska)
 static void matroska_parse_cues(MatroskaDemuxContext *matroska) {
     int i;
 
+    if (matroska->ctx->flags & AVFMT_FLAG_IGNIDX)
+        return;
+
     for (i = 0; i < matroska->num_level1_elems; i++) {
         MatroskaLevel1Element *elem = &matroska->level1_elems[i];
         if (elem->id == MATROSKA_ID_CUES && !elem->parsed) {