restore: sigreturn_restore: free core _after_ using it

Currently we have this:
	.......
	/* No longer need it */
	core_entry__free_unpacked(core, NULL);

	ret = prepare_itimers(pid, core, task_args);
	if (ret < 0)
		goto err;
	.......

So we're using ptr right after free-ing it.

Signed-off-by: Ruslan Kuprieiev <kupruser@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>