A fix for Bug#11901 "mysql_stmt_attr_set CURSOR_TYPE_READ_ONLY join in

subqry order by server crash": failing DBUG_ASSERT(curr_join == this)
when opening a cursor.
Ensure that for top-level join curr_join == join (always), 
and thus fix the failing assert.
curr_join is a hack to ensure that uncacheable subqueries can be
re-evaluated safely, and should be never different from main join
in case of top-level join.

sql/sql_select.h

@@ -366,6 +366,11 @@ class JOIN :public Sql_alloc
 	    !group_list);
   }
   bool change_result(select_result *result);
+  bool is_top_level_join() const
+  {
+    return (unit == &thd->lex->unit && (unit->fake_select_lex == 0 ||
+                                        select_lex == unit->fake_select_lex));
+  }
 };