Prevent stacktrace when DNS AAAA record is added

This patch fixes a stacktrace that is printed out when a IPv6
AAAA record with subnet prefix length (e.g. /64) is added.
The same error message as when IPv4 record with subnet prefix
length is used.

https://fedorahosted.org/freeipa/ticket/1115

ipalib/plugins/dns.py

@@ -113,7 +113,7 @@ def _create_zone_serial(**kwargs):
 def _validate_ipaddr(ugettext, ipaddr):
     try:
         ip = netaddr.IPAddress(ipaddr)
-    except netaddr.AddrFormatError:
+    except (netaddr.AddrFormatError, ValueError):
         return u'invalid address format'
     return None