ICU-23414 Fix null pointer dereference in TransliterationRule copy constructor on allocation failure

[UnLucky252]

Linked Jira issue

ICU-23414

Summary

In the TransliterationRule copy constructor (icu4c/source/i18n/rbt_rule.cpp),
the result of uprv_malloc for the segments array was passed to
uprv_memcpy without a NULL check.

Cause

On OOM uprv_malloc returns NULL; the subsequent uprv_memcpy is then
undefined behavior.

Fix

The constructor has no UErrorCode, so guard the uprv_memcpy with a
NULL check. On failure segments stays nullptr, which the rest of the
object already tolerates.

Testing

Existing tests pass (no behavior change on the success path). No new
test added — the OOM path requires allocator injection that is not
part of the standard test harness.

Notes

Found by static analysis (Svace, ISP RAS).

Checklist

• Required: Issue filed: ICU-23414
• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN 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
• Approver: Feel free to merge on my behalf