avcodec/dvbsubdec: Check entry_id

Fixes: randomly writing over the array end
Fixes: 1473/clusterfuzz-testcase-minimized-5768907824562176

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