Another attempt to fix the memory leak of mdev-5400.

sql/sql_select.cc

@@ -10533,6 +10533,7 @@ void JOIN::cleanup(bool full)
       {
 	tab->cleanup();
       }
+      table= 0;      
     }
     else
     {

sql/sql_union.cc

@@ -628,6 +628,7 @@ bool st_select_lex_unit::exec()
     {
       ha_rows records_at_start= 0;
       thd->lex->current_select= sl;
+      fake_select_lex->uncacheable|= sl->uncacheable;
 
       {
         set_limit(sl);