MDEV-5104 crash in Item_field::used_tables with broken order by

Analysis:
st_select_lex_unit::prepare() computes can_skip_order_by as TRUE.
As a result join->prepare() gets called with order == NULL, and
doesn't do name resolution for the inner ORDER clause. Due to this
the prepare phase doesn't detect that the query references non-exiting
function and field.
  
Later join->optimize() calls update_used_tables() for a non-resolved
Item_field, which understandably has no Field object. This call results
in a crash.

Solution:
Resolve unnecessary ORDER BY clauses to detect if they reference non-exising
objects. Then remove such clauses from the JOIN object.

sql/item_subselect.cc

@@ -2928,6 +2928,7 @@ int subselect_single_select_engine::prepare()
 		    select_lex->order_list.elements +
 		    select_lex->group_list.elements,
 		    select_lex->order_list.first,
+                    false,
 		    select_lex->group_list.first,
 		    select_lex->having,
 		    NULL, select_lex,