- The problem was that JOIN::save/restore_query_plan() did not save/restore parts of 
  the query plan that are located inside SJ_MATERIALIZATION_INFO structures. This could
  cause parts of one plan to be used with another, which led get_best_combination() to
  constructing non-sensical join plans (and crash).
  Fixed by saving/restoring SJM parts of the query plans.

- check_and_do_in_subquery_rewrites() will not set SUBS_MATERIALIZATION flag when it 
  records that the subquery predicate is to be converted into semi-join. 
  If convert_join_subqueries_to_semijoins() later decides not to convert to semi-join,
  let it set SUBS_MATERIALIZATION flag, if appropriate.