avcodec/tiertexseqv: set the fixed dimenasions, do not depend on the demuxer doing so

Fixes: out of array access
Fixes: 1348/clusterfuzz-testcase-minimized-6195673642827776

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/tiertexseqv.c

@@ -213,10 +213,15 @@ static int seqvideo_decode(SeqVideoContext *seq, const unsigned char *data, int
 static av_cold int seqvideo_decode_init(AVCodecContext *avctx)
 {
     SeqVideoContext *seq = avctx->priv_data;
+    int ret;
 
     seq->avctx = avctx;
     avctx->pix_fmt = AV_PIX_FMT_PAL8;
 
+    ret = ff_set_dimensions(avctx, 256, 128);
+    if (ret < 0)
+        return ret;
+
     seq->frame = av_frame_alloc();
     if (!seq->frame)
         return AVERROR(ENOMEM);