ICU-23418 Fix null pointer dereferences in SearchIterator constructors on allocation failure

[UnLucky252]

Linked Jira issue

ICU-23418

Summary

All four SearchIterator constructors (icu4c/source/i18n/search.cpp)
used the result of uprv_malloc(sizeof(USearch)) to initialize the
USearch fields without a NULL check.

Cause

On OOM uprv_malloc returns NULL; every field write is then undefined
behavior.

Fix

These are protected base-class constructors with no UErrorCode
(subclasses such as StringSearch already propagate
U_MEMORY_ALLOCATION_ERROR, and the destructor already guards for
nullptr). Guard the field initialization in each constructor with
if (m_search_ != nullptr). In the CharacterIterator constructor,
text.getText(m_text_) is moved before the guard so it always runs.

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-23418
• 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