Can't plant into scan in append mode

Here's an example:

```py
def body(carry, _):
  return harvest.sow(carry, name='x', mode='append', tag='tag'), None


harvest.plant(lambda x: jax.lax.scan(body, x, None, 5), tag='tag')({'x': 1}, 1)
```

This crashes with an `IndexError: list assignment index out of range`. The equivalent snippet with `mode='clobber'` works fine.