avformat/matroskadec: avcodec/tta: Set extradata_size to 22

Up until c4e0e314, the seek table has been included in the tta
extradata, so that the size of said extradata was 22 (the size of a TTA1
header) + 4 * number of frames. The decoder rejected anything below a
size of 30 and so the Matroska demuxer exported 30 byte long extradata,
of which only 18 were set (it ignores a CRC-32 and simply leaves it at
0). But this is unnecessary since said commit, so reduce the size to 22.

Furthermore, replace 30 by 22 in a comment about the extradata size in
libavcodec/tta.c.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/tta.c

@@ -129,7 +129,7 @@ static av_cold int tta_decode_init(AVCodecContext * avctx)
 
     s->avctx = avctx;
 
-    // 30bytes includes TTA1 header
+    // 22 bytes for a TTA1 header
     if (avctx->extradata_size < 22)
         return AVERROR_INVALIDDATA;