avcodec/cbs_h265_syntax_template: Check num_negative/positive_pics when inter_ref_pic_set_prediction_flag is set

Fixes: out of array access
Fixes: 20446/clusterfuzz-testcase-minimized-ffmpeg_BSF_HEVC_METADATA_fuzzer-5707770718584832

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