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

@@ -1373,8 +1373,10 @@ String *Item_func_database::val_str(String *str)
 
 String *Item_func_user::val_str(String *str)
 {
+  // TODO: make USER() replicate properly (currently it is replicated to "")
   THD *thd=current_thd;
-  if (str->copy((const char*) thd->user,(uint) strlen(thd->user)) ||
+  if (!(thd->user) || // for system threads (e.g. replication SQL 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;