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>

ssh/agent/server.go

@@ -541,6 +541,9 @@ func ServeAgent(agent Agent, c io.ReadWriter) error {
 			return err
 		}
 		l := binary.BigEndian.Uint32(length[:])
+		if l == 0 {
+			return fmt.Errorf("agent: request size is 0")
+		}
 		if l > maxAgentResponseBytes {
 			// We also cap requests.
 			return fmt.Errorf("agent: request too large: %d", l)