Fixes an error introduced in 22d3b6d9 (!294).
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.