src/clients/lib/xmmsclient/xmmsclient.c: fix -std=c23 build

gcc-15 switched to -std=c23 by default:

    https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=55e3bd376b2214e200fa76d12b67ff259b06c212

Without the change the build fails as:

    ../src/clients/lib/xmmsclient/xmmsclient.c:225:55: error: incompatible types when returning type ‘_Bool’ but ‘char *’ was expected
      225 |         x_api_error_if (!c, "with a NULL connection", false);
          |                                                       ^~~~~

src/clients/lib/xmmsclient/xmmsclient.c

@@ -222,7 +222,7 @@ xmmsc_disconnect_callback_set_full (xmmsc_connection_t *c,
 char *
 xmmsc_get_last_error (xmmsc_connection_t *c)
 {
-	x_api_error_if (!c, "with a NULL connection", false);
+	x_api_error_if (!c, "with a NULL connection", NULL);
 	return c->error;
 }