修复webclient GET request failed情况下内存双重释放问题

src/webclient.c

@@ -1398,11 +1398,13 @@ static int webclient_clean(struct webclient_session *session)
     if (session->host)
     {
         web_free(session->host);
+        session->host = RT_NULL;
     }
 
     if (session->req_url)
     {
         web_free(session->req_url);
+        session->req_url = RT_NULL;
     }
 
     session->content_length = -1;