avformat/matroskaenc: Write SeekHead when livestreaming

Commit 6fd300ac added support for WebM
Chunk livestreaming; in this case, both the header as well as each
Cluster is written to a file of its own, so that even if the AVIOContext
seems seekable, the muxer has to behave as if it were not. Yet one of
the added checks makes no sense: It ensures that no SeekHead is written
preliminarily (and hence no SeekHead is written at all) if the option
for livestreaming is set, although one should write the SeekHead in this
case when writing the Header. E.g. the WebM-DASH specification [1]
never forbids writing a SeekHead and in some instances (that don't apply
here) even requires it (if Cues are written after the Clusters).

[1]: https://sites.google.com/a/webmproject.org/wiki/adaptive-streaming/webm-dash-specificationSigned-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/matroskaenc.c

@@ -1896,7 +1896,7 @@ static int mkv_write_header(AVFormatContext *s)
     if (ret < 0)
         return ret;
 
-    if (!(s->pb->seekable & AVIO_SEEKABLE_NORMAL) && !mkv->is_live) {
+    if (!(s->pb->seekable & AVIO_SEEKABLE_NORMAL) || mkv->is_live) {
         ret = mkv_write_seekhead(pb, mkv, 0, avio_tell(pb));
         if (ret < 0)
             return ret;