Use the hostmask argument isProtected is given instead of trying to use a non-existant variable.

plugins/Protector/plugin.py

@@ -58,7 +58,7 @@ class Protector(callbacks.Plugin):
 
     def isProtected(self, irc, channel, hostmask):
         cap = ircdb.makeChannelCapability(channel, 'protected')
-        if ircdb.checkCapability(msg.prefix, cap):
+        if ircdb.checkCapability(hostmask, cap):
             self.log.debug('%s is protected on %s, it has %s.',
                            hostmask, channel, cap)
             return True