iptables plugin, utils_ignorelist: Fixed an off-by-one error each.

Those were introduced when unifying the string handling in commit 5f9ec13b in
cases where the exact length of the string to be copied is passed to sstrncpy
instead of the size of the destination buffer.

In case of the iptables plugin this prevented the table or chain name to match
correctly as the user configuration was truncated. In case of the ignorelist a
given regex was truncated.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>