avcodec/proresdec2: SKIP_BITS() does not work with len=32

Fixes: invalid shift
Fixes: 3482/clusterfuzz-testcase-minimized-5446915875405824

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