avcodec/flacdec: avoid undefined shift

Fixes: shift exponent 32 is too large for 32-bit type 'unsigned int'
Fixes: 4688/clusterfuzz-testcase-minimized-6572210748653568

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