Added a test case for bug #10124.
sql_select.h, item_subselect.cc, sql_select.cc:
  Fixed bug #10124.
  The copy method of the store_key classes can return
  STORE_KEY_OK=0, STORE_KEY_FATAL=1, STORE_KEY_CONV=2 now.
field.cc:
  Fixed bug #10124.
  When ussuing a warning the store methods return 2 instead of 1 now.