Fix nil pointer dereference for PEM certificates

According to the documentation
[`Decode`](https://pkg.go.dev/encoding/pem#Decode) returns nil if no pem
data is found.

This is possible if the certificate contains additional data which is
not part of the certificate. If this data occurs at the end the code
before this commit panics. This commit ensures that such data is
ignored. Data between two certificates in the chain is ignored as well.
Leading data leads to an error being returned.

Closes #359

internal/certificate/certdiff.go

@@ -61,6 +61,10 @@ func parseCertificates(cert string) ([]*x509.Certificate, error) {
 		var blk *pem.Block
 
 		blk, rest = pem.Decode(rest)
+		if blk == nil {
+			// No (further) PEM data found. We are done.
+			break
+		}
 		c, err := x509.ParseCertificate(blk.Bytes)
 		if err != nil {
 			return nil, fmt.Errorf("%s: %v", op, err)

internal/certificate/certdiff_internal_test.go

@@ -0,0 +1,39 @@
+package certificate
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/hetznercloud/terraform-provider-hcloud/internal/testsupport"
+	"github.com/stretchr/testify/assert"
+)
+
+// TestParseCertificates_CertificateChain tries to parse a chain of PEM
+// encoded certificates with interspersed and terminating data.
+// See: https://github.com/hetznercloud/terraform-provider-hcloud/issues/359
+func TestParseCertificates_CertificateChain(t *testing.T) {
+	pem1, _, err := testsupport.RandTLSCert("example.com")
+	assert.NoError(t, err)
+	cert1, err := parseCertificates(pem1)
+	assert.NoError(t, err)
+
+	pem2, _, err := testsupport.RandTLSCert("ca.example.com")
+	if !assert.NoError(t, err) {
+		return
+	}
+	cert2, err := parseCertificates(pem2)
+	if !assert.NoError(t, err) {
+		return
+	}
+
+	// Not really a certificate chain, but enough for our testing purposes
+	chain := strings.Join([]string{pem1, pem2}, "\nIntermediate data\n") + "Terminating data"
+	actual, err := parseCertificates(chain)
+	assert.NoError(t, err)
+
+	if !assert.Len(t, actual, 2) {
+		return
+	}
+	assert.Equal(t, cert1[0], actual[0])
+	assert.Equal(t, cert2[0], actual[1])
+}