ICU-23334 Fix null-deref and stale UErrorCode in break_iterator_fuzzer

[OwenSanzas]

Summary

Fix two issues in icu4c/source/test/fuzzer/break_iterator_fuzzer.cpp:

1. Null pointer dereference (line 57)

bi->setText(fuzzstr, status) dereferences bi without checking for nullptr. If utext_openUChars or createXxxInstance fails (e.g., due to OOM setting status to error), bi is nullptr and the call causes a SEGV.

Confirmed with AddressSanitizer:

ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000
The signal is caused by a READ memory access.
Hint: address points to the zero page.

Fix: Add null/error checks after utext_openUChars and after createXxxInstance, before calling bi->setText().

2. UErrorCode not reset before reuse (line 70)

The second round of createXxxInstance calls (lines 70-86) reuses status without resetting it to U_ZERO_ERROR. ICU's API protocol requires UErrorCode to be U_ZERO_ERROR before each call — if a previous call set it to error, subsequent ICU APIs silently return without executing. This makes the second phase of the harness effectively dead code whenever the first phase encounters any error.

Fix: Add status = U_ZERO_ERROR before the second phase.

Before/After Comparison (60-second fuzzing, AddressSanitizer)

Metric	Original	Fixed	Change
Edge cov	4272	4272	0%
Features	12601	12855	+2.0%
Corpus	1353	1421	+5.0%
Exec/s	3432	3561	+3.8%

Coverage is identical. No crashes in either version during 60-second run. The fixed version runs slightly faster.

Changes

Three minimal additions (no behavioral changes to the happy path):

1. Check utext_openUChars return value before use
2. Check bi for nullptr before calling setText
3. Reset status before second createXxxInstance round

[CLAassistant]

All committers have signed the CLA.