ICU-23443 Fix heap under-allocation and missing OOM check in VTimeZone/IZRule C wrappers

[UnLucky252]

Linked Jira issue

ICU-23443

Summary

Four C wrapper functions in icu4c/source/i18n/vzone.cpp and
icu4c/source/i18n/zrule.cpp allocated their output char16_t*
buffer with uprv_malloc(length) instead of
uprv_malloc(length * sizeof(char16_t)), and then copied
length bytes instead of length * sizeof(char16_t) bytes —
so the returned buffer was half the size implied by the
returned length. They also did not check the malloc result
for nullptr before memcpy.

Affected functions:

• vzone_write
• vzone_writeFromStart
• vzone_writeSimple
• izrule_getName

Cause

UnicodeString::length() returns the number of UTF-16 code units;
each unit is sizeof(char16_t) == 2 bytes. The malloc/memcpy used
that count as a byte count. On OOM uprv_malloc returns nullptr and
the subsequent memcpy is undefined behavior.

Fix

• Allocate and copy length * sizeof(char16_t) bytes.
• Check the malloc result; on failure set
  status = U_MEMORY_ALLOCATION_ERROR (where a UErrorCode&
  parameter is available) and reset the out-parameters.
• For the vzone_* functions also bail out early when the inner
  VTimeZone::write* call already set a failure.
• izrule_getName has no UErrorCode parameter (changing the
  signature would break ABI), so on OOM it can only signal by
  setting nameLength = 0 and leaving name = nullptr.

Testing

Existing tests pass (no behavior change on the success path beyond
returning the full UTF-16 buffer that callers always assumed).
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-23443
• 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