avcodec/ffv1dec: Check for bitstream end in decode_line()

Fixes: timeout
Fixes: 2971/clusterfuzz-testcase-6130678276030464

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