sel/bres-adns.c: Fix incorrect array bound.

GCC helpfully points out that `N(av)' does nothing very useful.  In
fact, I expect that it's almost always zero, which means that we don't
end up assembling addresses correctly.  And we never have done.

How embarrassing.

(cherry picked from commit 5086ffd7745337f5bee3c5c771cfcf1da71b03c3)