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.cc:
  - utilize tmp_join only if we evaluate an uncacheable subquery.
sql/sql_select.h:
  - implement JOIN::is_top_level_join()
tests/mysql_client_test.c:
  A test case for Bug#11901 "mysql_stmt_attr_set CURSOR_TYPE_READ_ONLY
   join in subqry order by server crash"