zile undo bug crash at scratch

[acoret]

undo work pretty well with other buff or file, but it will crash at
*scratch* buff.

```shell

zile
<C+x> b
<enter> // switch to scratch buffer
<C+/> // undo
<C+/>

```
zile will just crash.

undo will save action even after undo is invoked.
but revert_action may lead last_undop into null, but undo_save still
invoke last_undop->prepend without check, so it lead to crash


```patch
--- zile-2.6.2/src/undo.vala
+++ zile-2.6.2/src/undo.vala
@@ -49,12 +49,12 @@
if (type == SAVE_BLOCK) {
u.size = size;
u.text = cur_bp.get_region (new Region (o, o + osize));
u.unchanged = !cur_bp.modified;
}
- if (cur_bp.last_undop != null)
- cur_bp.last_undop->prepend (u);
+
+ cur_bp.last_undop->prepend (u);
}
```