x/crypto/ssh: empty plaintext packet causes panic

[rolandshoemaker]

When using AES-GCM or ChaCha20Poly1305, consuming a malformed packet which contains empty plaintext causes a panic, due to the assumption that there will always be at least one byte, containing the number of padding bytes.

[gopherbot]

Change https://golang.org/cl/368814 mentions this issue: ssh: don't assume packet plaintext size

[dhcgn]

This CVE 2021-43565 has a status of RESERVED by CVE, but should be public soon: https://nvd.nist.gov/vuln/detail/CVE-2021-43565