client/player: Fix scan-build warning

This fixes the following warning:

client/player.c:1775:25: warning: Dereference of null pointer
[core.NullDereference]
iov_append(&cfg->caps, preset->data.iov_base, preset->data.iov_len);
                        ^~~~~~~~~~~~~~~~~~~~~

client/player.c

@@ -1745,6 +1745,9 @@ static DBusMessage *endpoint_select_properties_reply(struct endpoint *ep,
 	DBusMessageIter iter;
 	struct endpoint_config *cfg;
 
+	if (!preset)
+		return NULL;
+
 	reply = dbus_message_new_method_return(msg);
 	if (!reply)
 		return NULL;