MDEV-32294 fix_fields() problem with inconsistent outer context paths

[mariadb-RexJohnston]

• [x] The Jira issue number for this PR is: MDEV-32294

Description

Exclude associated contexts when excluding SELECT_LEXs. These exclusions can happen when merging derived tables into the parent SELECT_LEX. Later, attempting to call fix_fields() on an Item that interacts with a merged SELECT_LEX should not be able to access a context associated with a merged select.

Release Notes

Usage of outer references in merged selects can cause problems, especially in prepared statements.

PR quality check

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