ssh: reset buffered packets after sending

Since encryption messes up the packets, the wrongly retained packets
look like noise and cause application protocol errors or panics in the
SSH library.

This normally triggers very rarely: the mandatory key exchange doesn't
have parallel writes, so this failure condition would be setup on the
first key exchange, take effect only after the second key exchange.

Fortunately, the tests against openssh exercise this. This change adds
also adds a unittest.

Fixes #18850.

Change-Id: I656c8b94bfb265831daa118f4d614a2f0c65d2af
Reviewed-on: https://go-review.googlesource.com/36056
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

ssh/handshake.go

@@ -314,7 +314,7 @@ write:
 				break
 			}
 		}
-		t.pendingPackets = t.pendingPackets[0:]
+		t.pendingPackets = t.pendingPackets[:0]
 		t.mu.Unlock()
 	}