fix panic in UnmarshalNebulaCertificate (#339)

This fixes a panic in UnmarshalNebulaCertificate when unmarshaling
a payload with Details set to nil.

Fixes: #332

cert/cert.go

@@ -61,6 +61,10 @@ func UnmarshalNebulaCertificate(b []byte) (*NebulaCertificate, error) {
 		return nil, err
 	}
 
+	if rc.Details == nil {
+		return nil, fmt.Errorf("encoded Details was nil")
+	}
+
 	if len(rc.Details.Ips)%2 != 0 {
 		return nil, fmt.Errorf("encoded IPs should be in pairs, an odd number was found")
 	}

cert/cert_test.go

@@ -499,6 +499,13 @@ func TestNebulaCertificate_Copy(t *testing.T) {
 	util.AssertDeepCopyEqual(t, c, cc)
 }
 
+func TestUnmarshalNebulaCertificate(t *testing.T) {
+	// Test that we don't panic with an invalid certificate (#332)
+	data := []byte("\x98\x00\x00")
+	_, err := UnmarshalNebulaCertificate(data)
+	assert.EqualError(t, err, "encoded Details was nil")
+}
+
 func newTestCaCert(before, after time.Time, ips, subnets []*net.IPNet, groups []string) (*NebulaCertificate, []byte, []byte, error) {
 	pub, priv, err := ed25519.GenerateKey(rand.Reader)
 	if before.IsZero() {