fix CT rule generation with ipfilter set

fix CT rule generation with ipfilter set

commit 255698f65192e736708f123d380bbed2aa8c3eac tried to prevent an
error from happening but wasn't to well thought out, perl's operator
precedence was overlooked.
The commit resulted effectively in:
if (my $ip = ($net->{ip} && $vmfw_conf->{options}->{ipfilter})) ...

But intended was:
if (defined(my $ip = $net->{ip}) && $vmfw_conf->{options}->{ipfilter}) ...

First one makes $ip always boolean true (1 in perl) if the if branch
is hit, and the seconds really has then the $ip value in it..

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>