branches/zip: row_upd_copy_columns(): Remove redundant code that was

accidentally added in r1591, which introduced dfield_t::ext in order
to make the merge sort of fast index creation support externally
stored columns.

Initially, I tried to allocate the bit for dfield_t::ext from
dfield_t::len by making the length 31 bits and mapping UNIV_SQL_NULL
to something that would fit in it.  Then I decided that it would be
too risky.  The redundant check was part of the mapping.  The
condition may have been dfield_is_null() initially.

This is similar to the redundant code in row_sel_fetch_columns() that
was noticed by Sergey Petrunya on the MySQL internals list and removed
in r6285.  As far as I can tell, there are no redundant UNIV_SQL_NULL
assignments remaining after this change.

row/row0upd.c

@@ -1344,9 +1344,6 @@ row_upd_copy_columns(
 		data = rec_get_nth_field(rec, offsets,
 					 column->field_nos[SYM_CLUST_FIELD_NO],
 					 &len);
-		if (len == UNIV_SQL_NULL) {
-			len = UNIV_SQL_NULL;
-		}
 		eval_node_copy_and_alloc_val(column, data, len);
 
 		column = UT_LIST_GET_NEXT(col_var_list, column);