sec: ValidateAtSignatureTime panics on nil Data (regression from d2666063) #217

Describe the bug

d266606 ("sec: improved certcache, certlist and expired cert validation") dropped the nil-Data branch from the exported ValidateAtSignatureTime expiry policy:

// before
if args.Data == nil || ValidateSigTime(args.Data, args.Cert) {
// after
if ValidateSigTime(args.Data, args.Cert) {

A nil Data is the documented case where the certificate itself is the object being validated and the signature-time check is deferred until the certificate is used to validate another packet. With the branch gone, the nil Data reaches ValidateSigTime, which dereferences it (data.Signature()) and the process dies with a nil pointer panic - a crash, not a validation result, for any consumer that registered this stock policy and hits the deferred path.

To Reproduce

Add the test file from the linked PR, then: go test ./std/security/ -run NilData -v

=== RUN   TestValidateAtSignatureTimeNilData
    Panic value: runtime error: invalid memory address or nil pointer dereference
--- FAIL: TestValidateAtSignatureTimeNilData (0.00s)
=== RUN   TestValidateSigTimeNilData
    Panic value: runtime error: invalid memory address or nil pointer dereference
--- FAIL: TestValidateSigTimeNilData (0.00s)
FAIL

Expected behavior

ValidateAtSignatureTime with a nil Data defers (accepts) as documented, and ValidateSigTime rejects nil input instead of panicking.

Environment

ndnd @ d51682b (main), Go 1.26, linux/amd64