Fix string formatting when port is not given.

If port is None, we would try to format an empty string with %d wich
fails. Fix it by using %s.

Reported-by: Iain Dalton <iain.dalton@gmail.com>
Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap/ui/UIBase.py

@@ -232,7 +232,7 @@ class UIBase:
         if s.verbose < 0: return
         displaystr = ''
         hostname = hostname if hostname else ''
-        port = "%d" % port if port else ''
+        port = "%s" % port if port else ''
         if hostname:
             displaystr = ' to %s:%s' % (hostname, port)
         s._msg("Establishing connection%s" % displaystr)