branches/zip: row_sel_fetch_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 redundant code was noticed by Sergey Petrunya on the MySQL
internals list.

row/row0sel.c

@@ -431,10 +431,6 @@ row_sel_fetch_columns(
 				data = rec_get_nth_field(rec, offsets,
 							 field_no, &len);
 
-				if (len == UNIV_SQL_NULL) {
-					len = UNIV_SQL_NULL;
-				}
-
 				needs_copy = column->copy_val;
 			}