Fix error case memory leak (ARMmbed#1537)

When panid query was received with missing TLV, scan_ptr was set to NULL without
pointer free. In some cases, that caused memory leak.

Author: Tero Heinonen

source/6LoWPAN/Thread/thread_management_server.c

@@ -857,6 +857,7 @@ static int thread_management_server_panid_query_cb(int8_t service_id, uint8_t so
     this->scan_ptr->timer = eventOS_timeout_ms(thread_panid_scan_timeout_cb, 500, this);// Delay for the confirm response message
     if (!this->scan_ptr->timer) {
         ns_dyn_mem_free(this->scan_ptr);
+        this->scan_ptr = NULL;
         response_code = COAP_MSG_CODE_RESPONSE_INTERNAL_SERVER_ERROR;
         goto error_exit;
     }
@@ -875,7 +876,6 @@ static int thread_management_server_panid_query_cb(int8_t service_id, uint8_t so
     }
     return -1;
 error_exit:
-    this->scan_ptr = NULL;
     if (request_ptr->msg_type == COAP_MSG_TYPE_CONFIRMABLE ){
         coap_service_response_send(this->coap_service_id, COAP_REQUEST_OPTIONS_NONE, request_ptr, response_code, COAP_CT_OCTET_STREAM, NULL, 0);
         return 0;