ssh/agent: prevent panic on malformed constraint

An attacker could supply a malformed Constraint that
would trigger a panic in a serving agent, effectively
causing denial of service.

Thank you to Jakub Ciolek for reporting this issue.

Fixes CVE-2025-47914
Fixes golang/go#76364

Change-Id: I195bbc68b1560d4f04897722a6a653a7cbf086eb
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/721960
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

ssh/agent/server.go

@@ -203,6 +203,9 @@ func parseConstraints(constraints []byte) (lifetimeSecs uint32, confirmBeforeUse
 	for len(constraints) != 0 {
 		switch constraints[0] {
 		case agentConstrainLifetime:
+			if len(constraints) < 5 {
+				return 0, false, nil, io.ErrUnexpectedEOF
+			}
 			lifetimeSecs = binary.BigEndian.Uint32(constraints[1:5])
 			constraints = constraints[5:]
 		case agentConstrainConfirm: