BUG#922254: Assertion `0' failed at item_cmpfunc.cc:5899: Item* Item_equal::get_first(JOIN_TAB*, Item*)
Fixed Item* Item_equal::get_first(JOIN_TAB *context, Item *field_item) to work correctly in the case where:
- context!= NO_PARTICULAR_TAB, it points to a table within SJ-Materialization nest
- field_item points to an item_equal that has a constant Item_field but does not have any fields
  from tables that are within semi-join nests.