mysql-test/r/subselect.result:
  Testcase for BUG#7885
mysql-test/t/subselect.test:
  Testcase for BUG#7885
sql/item_subselect.cc:
  Fix for BUG#7885: In Item_subselect::fix_fields, return error if engine->prepare fails.
  Also removed redundant code line.