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

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.