certificates: Retrieve serial numbers in canonical form

The x509 plugin retrieves serial numbers with two's complement
encoding whereas the openssl plugin partially returns them without
leading zeroes.

Serial numbers in X.509 certificates, X.509 CRL, X.509 attribute
certificates, OCSP Requests and OCSP responses are now returned in
canonical form without prepended zero octets.

src/libstrongswan/plugins/openssl/openssl_crl.c

@@ -1,6 +1,7 @@
 /*
  * Copyright (C) 2017 Tobias Brunner
  * Copyright (C) 2010 Martin Willi
+ * Copyright (C) 2022 Andreas Steffen
  *
  * Copyright (C) secunet Security Networks AG
  *
@@ -230,7 +231,7 @@ METHOD(crl_t, create_enumerator, enumerator_t*,
 METHOD(crl_t, get_serial, chunk_t,
 	private_openssl_crl_t *this)
 {
-	return this->serial;
+	return chunk_skip_zero(this->serial);
 }
 
 METHOD(crl_t, is_delta_crl, bool,
@@ -240,7 +241,7 @@ METHOD(crl_t, is_delta_crl, bool,
 	{
 		if (base_crl)
 		{
-			*base_crl = this->base;
+			*base_crl = chunk_skip_zero(this->base);
 		}
 		return TRUE;
 	}