ICU-23112 Add check for avoid underflow in MeasureUnit::getIdentifier

[wooffie]

Checklist

• [x] Required: Issue filed: ICU-23112
• [x] Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• [x] Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• [ ] Issue accepted (done by Technical Committee after discussion)
• [ ] Tests included, if applicable
• [ ] API docs and/or User Guide docs changed or added, if applicable

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• icu4c/source/i18n/measunit.cpp is different

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[markusicu]

https://unicode-org.atlassian.net/browse/ICU-23112?focusedCommentId=185435