MDEV-34976 Server crash report broken if Galera is not loaded

[temeo]

• The Jira issue number for this PR is: MDEV-34976

Description

The crash report terminates prematurely when Galera library was not loaded.

As a fix, check whether the provider is loaded before shutting down Galera connections.

How can this PR be tested?

No MTR test cases exist. Test by starting the server, signal the server to abort and observer crash report output.

Basing the PR against the correct MariaDB version

• This is a new feature or a refactoring, and the PR is based against the main 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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