avcodec/cbs_av1_syntax_template: Set seen_frame_header only after successfull uncompressed_header()

Fixes: assertion failure
Fixes: 19301/clusterfuzz-testcase-minimized-ffmpeg_BSF_AV1_FRAME_MERGE_fuzzer-5743212006473728

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 a2e4879432)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/cbs_av1_syntax_template.c

@@ -1500,8 +1500,6 @@ static int FUNC(frame_header_obu)(CodedBitstreamContext *ctx, RWContext *rw,
         else
             HEADER("Frame Header");
 
-        priv->seen_frame_header = 1;
-
 #ifdef READ
         start_pos = get_bits_count(rw);
 #else