support 'success' phase correctly

When the GSSAPI mechanism's process() function is invoked for the third time (on success) it must not attempt further processing.  Instead it should clean the context and return an empty response.

sleekxmpp/util/sasl/mechanisms.py

@@ -532,6 +532,9 @@ else:
                     result = kerberos.authGSSClientStep(self.gss, b64_challenge)
                     if result != kerberos.AUTH_GSS_CONTINUE:
                         self.step = 1
+                elif not challenge:
+                    kerberos.authGSSClientClean(self.gss)
+                    return b''
                 elif self.step == 1:
                     username = self.credentials['username']