BUG#861147: Assertion `fixed == 1' failed in Item_func_eq::val_int() with semijoin + materialization
- convert_subq_to_jtbm() didn't check that subuqery optimization was successful. If it wasn't (in this
  example because of @@max_join_size violation), it would proceed further and eventually crash when 
  trying to execute the un-optimized subquery.