avcodec/dvbsubdec: Check for duplicate regions in dvbsub_parse_page_segment()

Fixes: OOM
Fixes: 3051/clusterfuzz-testcase-minimized-5745818336231424

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