There were erroneous calls for charpos() in key_hashnr() and key_buf_cmp().
These functions are never called with prefix segments.

The charpos() calls were wrong. Before the change BNHL joins
- could return wrong result sets, as reported in MDEV-34417
- were extremely slow for multi-byte character sets, because
  the hash was calculated on string prefixes, which increased
  the amount of collisions drastically.

This patch fixes the wrong result set as reported in MDEV-34417,
as well as (partially) the performance problem reported in MDEV-34352.