Fix SNAT never being added because of exception

Some firewall rule object (iptc) do not have a parameter
attribute, which results in an exception being triggered,
and the mailcow SNAT rule to never be created.

Firewall rules that trigger such exception are:
- -A POSTROUTING -s 192.168.122.0/24 -d 224.0.0.0/24 -j RETURN

This commit just verify attribute presence, and skip the rule
properly instead of triggering an exception.

data/Dockerfiles/netfilter/server.py

@@ -366,6 +366,8 @@ def get_snat4_rule():
           chain.insert_rule(new_rule)
         else:
           for position, rule in enumerate(chain.rules):
+            if not hasattr(rule.target, 'parameter'):
+                continue
             match = all((
               new_rule.get_src() == rule.get_src(),
               new_rule.get_dst() == rule.get_dst(),