avcodec/lagarith: Detect end of input in lag_decode_line() loop

Fixes: timeout
Fixes: 2933/clusterfuzz-testcase-5124990208835584

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