avcodec/proresdec2: Check bits in DECODE_CODEWORD(), fixes invalid shift

Fixes: runtime error: shift exponent 42 is too large for 32-bit type 'unsigned int'
Fixes: 3410/clusterfuzz-testcase-minimized-5313377960198144

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