avcodec/lagarith: disable lag_decode_zero_run_line() and ask for a sample

The code seems to have never been tested
fixing it should be quite easy but needs a sample/testcase

Fixes use of uninitialized memory
Fixes: msan_uninit-mem_7f9a862dfabf_413_2889_assassin_OL.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
