avcodec/flacdec: reduce limit for golomb so that the max value does not overflow

Fixes: runtime error: left shift of 32 by 26 places cannot be represented in type 'int'

Fixes: 628/clusterfuzz-testcase-6187747641393152
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
