OSV-2017-90 Use-of-uninitialized-value in ComprDataIO::UnpRead

OSS-Fuzz report: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=4283 Crash type: Use-of-uninitialized-value Crash state: ComprDataIO::UnpRead Unpack::UnpReadBuf Unpack::Unpack15...