Fixed an error in the creation of REF access method for materialized

semi-join, where the the REF buffer format was mistaken to be in
record format instead of key format. The error was that the null
byte for all fields of the record was in the front of the buffer,
and not before each field data.

sql/opt_subselect.cc

@@ -2296,7 +2296,7 @@ bool setup_sj_materialization(JOIN_TAB *tab)
                                       use that information instead.
                                    */
                                    cur_ref_buff + null_count,
-                                   null_count ? tab_ref->key_buff : 0,
+                                   null_count ? cur_ref_buff : 0,
                                    cur_key_part->length, tab_ref->items[i]);
       cur_ref_buff+= cur_key_part->store_length;
     }