avcodec/pictordec: Fix runtime error: left shift of 805306368 by 2 places cannot be represented in type 'int'

Fixes: 823/clusterfuzz-testcase-6727060074528768

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