ninfod: Fix off-by-one error to check possible programming error (again).

This bug was once fixed by commit 70bcae85 ("ninfod: Fix off-by-one
error to check possible programming error."), but unfotunately, it was
re-introduced by commit 9a2a2664 ("ninfod: Introduce ARRAY_SIZE macro
for counting number of elements in an array.").

Bug noticed by Jan Synacek <jsynacek@redhat.com>.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>