MDEV-22855 Assertion !field->prefix_len || field->fixed_len == field->prefix_len failed in btr_node_ptr_max_size

[Thirunarayanan]

• The Jira issue number for this PR is: MDEV-22855

Description

Problem:

• InnoDB wrongly calulates the record size in btr_node_ptr_max_size() when prefix index of
  the column has to be stored externally.

Fix:

• InnoDB should add the maximum field size to record size when the field is a fixed length one.

How can this PR be tested?

./mtr innodb.innodb-index

Basing the PR against the correct MariaDB version

• This is a new feature and the PR is based against the latest MariaDB development branch.
• This is a bug fix and the PR is based against the earliest maintained branch in which the bug can be reproduced.

PR quality check

• I checked the CODING_STANDARDS.md file and my PR conforms to this where appropriate.
• For any trivial modifications to the PR, I am ok with the reviewer making the changes themselves.

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[dr-m]

Thank you. The fix looks OK, but I would like to clarify it a little, because I do not remember where exactly the decision to store long CHAR as VARCHAR has been implemented.