Discovered in BF-5146 fuzzer test. We can dereference MigrationDestionationManager::_sessionId, a boost::optional<MigrationSessionId>, when it is unset: https://github.com/mongodb/mongo/blob/ed80d2d3c5735c424e5c1cd58169077327976d2e/src/mongo/db/s/migration_destination_manager.cpp#L356

Backports are not needed, as the log message in which _sessionId is dereferenced is new as of v3.5