avformat/matroskadec: ProjectionPrivate is optional on Equirectangular projections

This reflects a recent change to the spec draft.
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/matroskadec.c

@@ -1913,8 +1913,6 @@ static int mkv_parse_video_projection(AVStream *st, const MatroskaTrack *track)
 
     switch (track->video.projection.type) {
     case MATROSKA_VIDEO_PROJECTION_TYPE_EQUIRECTANGULAR:
-        if (track->video.projection.private.size < 4)
-            return AVERROR_INVALIDDATA;
         projection = AV_SPHERICAL_EQUIRECTANGULAR;
         break;
     case MATROSKA_VIDEO_PROJECTION_TYPE_CUBEMAP: