support: make sure loop_add_io is closes the fd when asked

When the close argument is true, loop_add_io is supposed to close the fd
even when an error occurs.

This makes it possible to have spa_steal_fd() work even in the case
of errors.

spa/plugins/support/loop.c

@@ -922,6 +922,8 @@ error_exit_free:
 	free(source);
 	errno = -res;
 error_exit:
+	if (close)
+		spa_system_close(impl->system, fd);
 	return NULL;
 }