ICU-23444 Fix null pointer dereference and leak in SpoofData::reserveSpace on uprv_realloc failure

[UnLucky252]

Linked Jira issue

ICU-23444

Summary

In SpoofData::reserveSpace (icu4c/source/i18n/uspoof_impl.cpp),
the result of uprv_realloc(fRawData, fMemLimit) was assigned
directly back to fRawData and immediately dereferenced
(fRawData->fLength = ...).

Cause

Two problems:

1. Null pointer dereference on OOM. uprv_realloc returns
   nullptr on allocation failure; the next write through that
   nullptr is undefined behavior.
2. Memory leak via the standard realloc gotcha. When
   realloc(p, n) fails it returns nullptr but does not free
   the original buffer. Assigning the result directly back to
   fRawData overwrites the live pointer with nullptr and loses
   the original allocation.

Fix

Take the uprv_realloc result into a temporary, check for nullptr,
set status = U_MEMORY_ALLOCATION_ERROR via the existing
UErrorCode& parameter and return without touching fRawData. On
success, assign the new pointer back to fRawData.

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-23444
• 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