avcodec/pngdec: Fix runtime error: left shift of 152 by 24 places cannot be represented in type 'int'

Fixes: 666/clusterfuzz-testcase-6581447227867136

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