avcodec/dvbsubdec: Check pixel buffer size constraint from ETSI EN 300 743 V1.3.1

Fixes: OOM
Fixes: 2143/clusterfuzz-testcase-minimized-5482288060039168

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