avcodec/dvdsubdec: Fix runtime error: left shift of 242 by 24 places cannot be represented in type 'int'

Fixes: 1080/clusterfuzz-testcase-5353236754071552

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