valgrind reported error: fixed uninitialised variables. (ARMmbed#1747)

Author: deepakvenugopal

source/6LoWPAN/Thread/thread_management_client.c

@@ -146,6 +146,8 @@ void thread_management_client_init(int8_t interface_id)
 
     if (this) {
         this->network_data_set_cb_ptr = NULL;
+        this->router_id_release_cb_ptr = NULL;
+        this->neighbor_discovery_cb_ptr = NULL;
         this->router_id_cb_ptr = NULL;
         this->interface_id = interface_id;
         this->coap_asd_msg_id = 0;