Only store established connections in the pool

If a connection has not been fully established (i.e. authentication has
been completed) then it should not be considered as a connection pool
candidate.

server/modules/protocol/maxscaled/maxscaled.c

@@ -185,7 +185,8 @@ MXS_MODULE* MXS_CREATE_MODULE()
         NULL,                           /**< Authentication                */
         NULL,                           /**< Session                       */
         mxsd_default_auth,              /**< Default authenticator         */
-        NULL                            /**< Connection limit reached      */
+        NULL,                           /**< Connection limit reached      */
+        NULL
     };
 
     static MXS_MODULE info =