encoding: Allow ke_payload_create_from_diffie_hellman() to fail

src/libcharon/encoding/payloads/ke_payload.h

@@ -73,7 +73,7 @@ ke_payload_t *ke_payload_create(payload_type_t type);
  *
  * @param type		PLV2_KEY_EXCHANGE or PLV1_KEY_EXCHANGE
  * @param dh		diffie hellman object containing group and key
- * @return 			ke_payload_t object
+ * @return 			ke_payload_t object, NULL on error
  */
 ke_payload_t *ke_payload_create_from_diffie_hellman(payload_type_t type,
 													diffie_hellman_t *dh);