ssh/agent: add checking for empty SSH requests

Previously empty SSH requests would cause a panic.

Change-Id: I8443fee50891b3d2b3b62ac01fb0b9e96244241f
GitHub-Last-Rev: 64f00d2bf2ee722f53e68b6bd4f70c722d7694bd
GitHub-Pull-Request: golang/crypto#58
Reviewed-on: https://go-review.googlesource.com/c/140237


Reviewed-by: Han-Wen Nienhuys <hanwen@google.com>
Run-TryBot: Han-Wen Nienhuys <hanwen@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>