Fix crash caused by buggy UPnP

gaierror: [Errno -2] Name or service not known
Traceback (most recent call last):
 File "re6st/cli/node.py", line 271, in main
   remote_gateway, config.disable_proto, config.neighbour)
 File "re6st/tunnel.py", line 663, in __init__
   cache, cert, address)
 File "re6st/tunnel.py", line 236, in __init__
   self._updateCountry(address)
 File "re6st/tunnel.py", line 643, in _updateCountry
   family, ip = resolve(*address)
 File "re6st/tunnel.py", line 30, in resolve
   for x in socket.getaddrinfo(ip, port, family, 0, proto))

where ip is '-a'

re6st/upnpigd.py

@@ -48,16 +48,14 @@ class Forwarder(object):
         t.append((self.next_refresh, self.refresh))
 
     def checkExternalIp(self, ip=None):
-        if ip:
+        if not ip:
+            ip = self.refresh()
         try:
             socket.inet_aton(ip)
         except socket.error:
-                ip = None
-        else:
-            ip = self.refresh()
-        # If port is None, we assume we're not NATed.
-        return socket.AF_INET, [(ip, str(port or local), proto)
-            for local, proto, port in self._rules] if ip else ()
+            ip = ()
+        return socket.AF_INET, ip and [(ip, str(port or local), proto)
+            for local, proto, port in self._rules]
 
     def addRule(self, local_port, proto):
         self._rules.append([local_port, proto, None])