kubevirt / containerized-data-importer / #6245

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Fix zstd decompression buffer corruption in CDI (#4278)

Fixes data corruption issue when trying to import a zst image.
Essentially the issue boils down to how cdi finds out which compressed
format the data is inside matchHeaders.

1) `matchHeader` reads 512 bytes from the raw network reader into fr.buf
2) It detects zst and pushes a zst reader to the reader stack
3) `matchHeader` uses the zst reader to read the first zstd block which
   in the case of my image was only 100b long. It then saves the decompressed data of 512 bytes back into `fr.buf` but we only read part of the original fr.buf.
   This causes us to overwrite compressed data with decompressed data.
4) The next read from the zst reader then tries reading decompressed
   data as a zst header and sees a very high window size leading to the
   error `failed to pull image: window size exceeded`

Signed-off-by: Trevor Moyer <trevormoyer@google.com>
Co-authored-by: Trevor Moyer <trevormoyer@google.com>

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