avcodec/xan: Check for bitstream end in xan_huffman_decode()

Fixes: Timeout
Fixes: 3707/clusterfuzz-testcase-6465922706440192

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