"It is a requirement of bitstream conformance that num_y_points is less than or equal to 14."

Fixes: index 24 out of bounds for type 'uint8_t [24]'
Fixes: 19282/clusterfuzz-testcase-minimized-ffmpeg_BSF_AV1_FRAME_MERGE_fuzzer-5747424845103104

Note, also needs a23dd336

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: jamrial
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>