Fix bug #21192. Thanks to John Darrington for review.

* casereader.c (casereader_read): Decrement case_cnt before
calling the casereader's "read" member function, so that we
interact properly with lazy_casereader.

* datasheet.c: Add regression test for above bug fix.
(clone_datasheet): New function.
(lazy_callback): New function.
(check_datasheet_casereader): New function.
(check_datasheet): Check datasheet contents are reported correctly
through an ordinary casereader and a lazy casereader.
(clone_model): Use clone_datasheet.