Purge crashes on a BLOB with a corrupted external-reference length

After ALTER TABLE ... IMPORT TABLESPACE imports a tablespace whose BLOB external-referencelength (BTR_EXTERN_LEN)
has been corrupted. Specifically its unused high 4 bytes (InnoDB only ever uses the low 4 bytes as the real length; the high bytes are documented as "currently unused"). Import itself succeeds,
and the row reads back fine.

The problem shows up later: once that row is deleted (or updated, triggering an old-version purge) and purge goes to free the BLOB's external pages, btr_free_externally_stored_field() hits a debug assertion

Test case refer this one MDEV-41942