avcodec/jpeglsdec: Check for end of bitstream in ls_decode_line()

Fixes: 1773/clusterfuzz-testcase-minimized-4832523987189760

Fixes: Timeout

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