obu: Initialize off_before_idx[0], fix MSVC warning about uninitialized use
obu: Initialize off_before_idx[0], fix MSVC warning about uninitialized use

MSVC warns that off_before_idx[0] can be unsed uninitialized
in the line "off_before_idx[1] = off_before_idx[0];" further below.

videolan/dav1d