lf mask: calculate vertical mask correctly
Fixes an error introduced in 22d3b6d9 (!294 (merged)). Fixes an use of uninitialized value in loop_filter_v_sb128y_c with clusterfuzz-testcase-minimized-dav1d_fuzzer-5682084585144320. The original sample in 22d3b6d9 clusterfuzz-testcase-minimized-dav1d_fuzzer-5691087507685376 shows no regression. Credits to oss-fuzz.
Edited by Janne Grunau