ICU-22884 Fix assertion and Heap-use-after-free

[FrankYFTang]

Issue in icu_76::message2::MessageFormatter::~MessageFormatter

Checklist

• Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22884
• Required: The PR title must be prefixed with a JIRA Issue number.
• Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• Required: Each commit message must be prefixed with a JIRA Issue number.
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable

[FrankYFTang]

@catamorphism

[catamorphism]

Thanks, LGTM!