Fix: zstd decompression is truncated to the first frame in the stream

[cormacrelf]

This fails with 131072 bytes read, which is coincidentally zstd's max block size. Bug discovered in https://github.com/facebook/buck2/pull/1103.

Clearly poll_read is returning "ready" with 0 bytes read when it should still be pending. I have not done a bisect to find when this bug comes from, but you could do it with a script to apply this commit as a patch at each bisect eval point.