Fix a bug in `BufferedReader::ReadHintSlow()`: subtract `available_length` from the `length` to read, like in `PullSlow()`. Also. there is no need to limit that to the buffer size, because the buffer was just resized to at least the required size.
Fix a bug in `BufferedReader::ReadHintSlow()`: subtract `available_length` from
the `length` to read, like in `PullSlow()`. Also. there is no need to limit that
to the buffer size, because the buffer was just resized to at least the required
size.

Actually the implementation becomes the same as `PullSlow()`, which is not
surprising: the buffer is flat. Just call `PullSlow()`.
PiperOrigin-RevId: 421755827

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