Problem with read after flush/close

[mattias-lundell]

Found something that seems like a regression. The use case is that you have an underlying buffer wrapped by the lz4 writer that you write to a bit and then flush/close, then you open a new writer around the buffer and write some more. This works well on
github.com/pierrec/lz4 but it fails on github.com/pierrec/lz4/v4. See the sandbox links for example.

ok: https://go.dev/play/p/39movQ38d3J

fail: https://go.dev/play/p/xKGvNhe9xcj