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/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
