avcodec/h264_parse: Check picture structure when initializing weight table

Fixes: runtime error: index 49 out of bounds for type 'int [48][2][2]'
Fixes: 2159/clusterfuzz-testcase-minimized-5267945972301824

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