avcodec/cbs_av1_syntax_template: reset seen_frame_header on sequence
headers

With this change CBS and the decoder appear to be in sync.

Fixes: division by 0

Fixes: 501794431/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AV1_fuzzer-4792576644546560
Fixes: 501898692/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AV1_fuzzer-4772278394224640
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit fd290e2fcdd431bdce5ae7f308d22e7eaa3afa96)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>