* Fix the crash: IN-to-EXISTS rewrite causes an error (and so
  JOIN::optimize() fails with an error, too), don't call
  update_used_tables(). Terminate the query execution instead.

* Fix the cause of the error in the IN-to-EXISTS rewrite: don't do
  the rewrite if doing it will cause an error of this kind:
  This version of MariaDB doesn't yet support 'SUBQUERY in ROW in left
  expression of IN/ALL/ANY'

* Fix another issue exposed by this testcase:
  JOIN::setup_subquery_caches() may be invoked before any select has
  saved its query plan, and will crash because none of the SELECTs
  has called create_explain_query_if_not_exists() to create the Explain
  Data Structure for this SELECT.

TODO: When merging this to 10.2, remove the poorly-placed call to
create_explain_query_if_not_exists made by fix for M_D_E_V-16153