Backporting the changeset below from 4.0, because a customer hits

the bug with 3.23.
ChangeSet@1.1416.113.1, 2003-03-22 15:22:59+01:00, guilhem@mysql.com
  Fix for #178  Replicating INSERT VALUES(USER()) crashes (SEGV) the slave
      Now it does not SEGV, but USER() is still badly replicated
      (it is replicated to ""), which is a lower priority bug.

sql/item_strfunc.cc

@@ -1094,7 +1094,8 @@ String *Item_func_database::val_str(String *str)
 String *Item_func_user::val_str(String *str)
 {
   THD *thd=current_thd;
-  if (str->copy((const char*) thd->user,(uint) strlen(thd->user)) ||
+  if (!(thd->user) || // for system threads (e.g. replication thread)
+      str->copy((const char*) thd->user,(uint) strlen(thd->user)) ||
       str->append('@') ||
       str->append(thd->host ? thd->host : thd->ip ? thd->ip : ""))
     return &empty_string;