avcodec/dvdsubdec: Check for fully transparent rectangles earlier

Testcase with large transparent rectangles changes from 67 sec to 3 sec decode time

Fixes: Timeout
Fixes: 8728/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_DVDSUB_fuzzer-5190088756559872

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg


Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>