mem-cred: Add a method to unify certificate references, without adding it

In contrast to add_cert_ref(), get_cert_ref() does not add the certificate to
the set, but only finds a reference to the same certificate, if found.

src/libstrongswan/credentials/sets/mem_cred.h

@@ -58,6 +58,18 @@ struct mem_cred_t {
 	certificate_t *(*add_cert_ref)(mem_cred_t *this, bool trusted,
 								   certificate_t *cert);
 
+	/**
+	 * Get an existing reference to the same certificate.
+	 *
+	 * Searches for the same certficate in the set, and returns a reference
+	 * to it, destroying the passed certificate. If the passed certificate
+	 * is not found, it is just returned.
+	 *
+	 * @param cert			certificate to look up
+	 * @return				the same certificate, potentially different instance
+	 */
+	certificate_t* (*get_cert_ref)(mem_cred_t *this, certificate_t *cert);
+
 	/**
 	 * Add an X.509 CRL to the credential set.
 	 *