Reference: ETSI EN 300 743 V1.2.1  7.2.2 Region composition segment

Fixes: Timeout
Fixes: 13325/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DVBSUB_fuzzer-5143979392237568

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