Fix `Chain::BlockIterator::CharIndexInChain()` when the iterator is `blocks().cend()`.
Fix `Chain::BlockIterator::CharIndexInChain()` when the iterator is
`blocks().cend()`.

This manifested as crashes in `ReaderFactory`, which is the only user of
`ChainIndexInChain()`.

PiperOrigin-RevId: 407577713

google/riegeli,google/riegeli,google/riegeli,google/riegeli