ICU-22962 Fix int32 overflow in Chinese Calendar

[FrankYFTang]

Fix int32 overflow found in https://g-issues.oss-fuzz.com/issues/383884624

Checklist

• Required: Issue filed: ICU-22962
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-1234 Fix xyz"
• 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. 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

[jira-pull-request-webhook]

Notice: the branch changed across the force-push!

• icu4c/source/test/intltest/caltest.cpp is different

View Diff Across Force-Push

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[richgillam]

LGTM