avcodec/lagarith: reenable buggy lag_decode_zero_run_line()

This is to prevent regressions in case the old code was able to partly
decode frames (no way to say without a testcase)
Add a memset to prevent use of uninitialized memory until we have a
testcase and can test/fix it

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
