fix of the small bug in UNION's

mysql-test/r/heap_btree.result

@@ -73,8 +73,8 @@ type=heap;
 insert into t1 values (1,1),(2,2),(1,3),(2,4),(2,5),(2,6);
 select * from t1 where x=1;
 x	y
-1	1
 1	3
+1	1
 select * from t1,t1 as t2 where t1.x=t2.y;
 x	y	x	y
 1	1	1	1

sql/sql_union.cc

@@ -109,7 +109,7 @@ int st_select_lex_unit::prepare(THD *thd, select_result *result)
   if (prepared)
     DBUG_RETURN(0);
   prepared= 1;
-    
+  union_result=0;
   describe=(first_select()->options & SELECT_DESCRIBE) ? 1 : 0;
   res= 0;
   found_rows_for_union= false;
@@ -309,10 +309,12 @@ int st_select_lex_unit::exec()
 int st_select_lex_unit::cleanup()
 {
   DBUG_ENTER("st_select_lex_unit::cleanup");
+  if (union_result)
+  {
     delete union_result;
     free_tmp_table(thd,table);
     table= 0; // Safety
-  
+  }
   List_iterator<JOIN*> j(joins);
   JOIN** join;
   while ((join= j++))