avcodec/pictordec: Fix runtime error: left shift of 64 by 25 places cannot be represented in type 'int'

Fixes: 724/clusterfuzz-testcase-6738249571631104

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