Fix collator_rulebased_fuzzer: stop masking errors by resetting UErrorCode

[OwenSanzas]

Summary

After each ICU API call, the fuzzer resets status = U_ZERO_ERROR and continues execution regardless of whether the previous call failed. This masks errors and allows the fuzzer to call subsequent APIs on objects in an error state.

For example, if getVariableTop(status) fails, the fuzzer resets status and proceeds to call getCollationKey(), getRules(), etc. on a potentially invalid collator state.

Fix: Check U_FAILURE(status) after each API call and return early on error, instead of blindly resetting the error code.

Coverage comparison (60 seconds, empty seed corpus, ASan, libFuzzer)

Metric	Original	Fixed	Change
Edge coverage	3509	3703	+5.53% (+194 edges)
Feature coverage	10231	11487	+12.28% (+1256 features)

Note: The edge coverage improvement may partially be due to fuzzer randomness, but confirms the fix does not regress coverage.