avcodec/exr: fix undefined shift in pxr24_uncompress()

Fixes: runtime error: left shift of 255 by 24 places cannot be represented in type 'int'
Fixes: 3787/clusterfuzz-testcase-minimized-5728764920070144

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
