btr_discard_only_page_on_level(): Attempt to read the MDEV-15562 metadata
record from the leaf page, not the root page. In the root, the leftmost
(in this case, the only) node pointer would look like a metadata record.

This corruption bug was introduced in
commit 0e5a4ac2 (MDEV-15562).
The scenario is rare: a column was dropped instantly or the order of
columns was changed instantly, and then the table became empty in such
a way that in the last step, the root page had one child page.
Normally, a non-leaf B-tree page would always contain at least 2 children.