OSV-2020-845 Use-of-uninitialized-value in isMatchAtCPBoundary

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=19497 Crash type: Use-of-uninitialized-value Crash state: isMatchAtCPBoundary ustrFindFirst66 icu66::UnicodeString::indexOf...