This fix does not remove the underlying cause of the assertion
failure. It just works around the problem, allowing a corrupted
secondary index to be fixed by DROP INDEX and CREATE INDEX (or in the
worst case, by re-creating the table).

ibuf_delete(): If the record to be purged is the last one in the page
or it is not delete-marked, refuse to purge it. Instead, write an
error message to the error log and let a debug assertion fail.

ibuf_set_del_mark(): If the record to be delete-marked is not found,
display some more information in the error log and let a debug
assertion fail.

row_undo_mod_del_unmark_sec_and_undo_update(),
row_upd_sec_index_entry(): Let a debug assertion fail when the record
to be delete-marked is not found.

buf_page_print(): Add ut_ad(0) so that corruption will be more
prominent in stress testing with debug binaries. Add ut_ad(0) here and
there where corruption is noticed.

btr_corruption_report(): Display some data on page_is_comp() mismatch.

btr_assert_not_corrupted(): A wrapper around btr_corruption_report().
Assert that page_is_comp() agrees with the table flags.

rb:911 approved by Inaam Rana