OSV-2021-1236 UNKNOWN READ in icu_70::UCharsTrie::branchNext

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=38390 Crash type: UNKNOWN READ Crash state: icu70::UCharsTrie::branchNext icu70::UCharsTrie::firstForCodePoint icu70::CollationIterator::nextCE32FromContraction...