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.
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