ICU-22764 Fix gendict memory safety in toml uchars mode

[sffc]

Checklist

• [x] Required: Issue filed: https://unicode-org.atlassian.net/browse/ICU-22764
• [x] Required: The PR title must be prefixed with a JIRA Issue number.
• [x] Required: The PR description must include the link to the Jira Issue, for example by completing the URL in the first checklist item
• [x] Required: Each commit message must be prefixed with a JIRA Issue number.
• [x] Issue accepted (done by Technical Committee after discussion)
• [ ] Tests included, if applicable
• [ ] API docs and/or User Guide docs changed or added, if applicable

[sffc]

CC @Manishearth

[markusicu]

@sffc ticket accepted