avcodec/h264_metadata_bsf: Check nb_units before accessing the first in h264_metadata_update_fragment()

Fixes: null pointer dereference
Fixes: 29835/clusterfuzz-testcase-minimized-ffmpeg_BSF_H264_METADATA_fuzzer-4712125383704576

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