Ignore public folders, fixed error message in logs

kopano-spamd.py

@@ -49,10 +49,11 @@ class Checker(object):
         if item.message_class == 'IPM.Note':
             spamstatus = item.header(self.spamheader)
             if spamstatus is not None:
-                if item.folder == item.store.user.junk and not spamstatus.lower().startswith('yes'):
-                    self.learn(item)
-                if item.folder == item.store.user.inbox and spamstatus.lower().startswith('yes'):
-                    self.unlearn(item)
+                if item.store.user:  # skip public stores
+                    if item.folder == item.store.user.junk and not spamstatus.lower().startswith('yes'):
+                        self.learn(item)
+                    if item.folder == item.store.user.inbox and spamstatus.lower().startswith('yes'):
+                        self.unlearn(item)
 
     def learn(self, item):
         with log_exc(self.log):