fix for bug #3974 ("SHOW FULL PROCESSLIST" crashes the embedded server)

server crashed checking thd->priv_user[0] and thd->priv_user is NULL
if NO_EMBEDDED_ACCESS_CHECKS is on.
Now i set it to be the same as thd->user


libmysqld/lib_sql.cc:
  now priv_user won't be NULL

libmysqld/lib_sql.cc

@@ -511,6 +511,7 @@ int check_embedded_connection(MYSQL *mysql)
   thd->host= (char*)my_localhost;
   thd->host_or_ip= thd->host;
   thd->user= my_strdup(mysql->user, MYF(0));
+  thd->priv_user= thd->user;
   return check_user(thd, COM_CONNECT, NULL, 0, thd->db, true);
 }