avcodec/pictordec: runtime error: left shift of 15 by 28 places cannot be represented in type 'int'

Fixes: 898/clusterfuzz-testcase-6149765467209728

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