Fix possible truncated output in ipset output buffer handling

Fix possible truncated output in ipset output buffer handling

Omri Bahumi and Yoni Lavi discovered that due to the inproper
handling of the ipset output buffer, the output may be truncated.
So for example in an "ipset save" output, instead of 192.168.0.0/24,
just 192.168.0.0 printed. If one use "ipset save" and then "ipset restore"
to restore the sets, this may lead to wrong firewall rules at the end.

The patch fixes the bug in the ipset code.