reveal used macs only once

without this, messages get larger the longer an otr session stays
active. this leads to anti-spam measures being triggered (on ICQ for
example) and is also the cause for the infamous bug mmb/weechat-otr#17

src/potr/crypt.py

@@ -241,6 +241,9 @@ def createDataMessage(self, message, flags=0, tlvs=None):
         msg = proto.DataMessage(flags, self.ourKeyid-1, self.theirKeyid,
                 long_to_bytes(self.ourDHKey.pub), sess.sendctr.byteprefix(),
                 encmsg, b'', b''.join(self.savedMacKeys))
+
+        self.savedMacKeys = []
+
         msg.mac = SHA1HMAC(sess.sendmac, msg.getMacedData())
         return msg