avcodec/flacdec: Return error code instead of 0 for failures

Fixes: infinite loop
Fixes: 1418/clusterfuzz-testcase-minimized-5934472438480896

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