[BUG] Setitem on struct columns picks out incorrect pieces when indexing of rvalue is not from zero.

**Describe the bug**

```python
import cudf

s = cudf.Series([{"a": 1}, {"a": 2}, {"a": 3}])
g = cudf.Series([{"a": 4}, {"a": 5}, {"a": 6}])

s.iloc[1:2] = g.iloc[2:3]

s.iloc[1:2]
# 1 {'a': 4}
# dtype: struct

g.iloc[2:3]
# 2 {'a': 6}
# dtype: struct

# An even more fun:

x = g.iloc[2:3].copy(deep=True)

s.iloc[1] = x.iloc[0]

s.iloc[1] # => {'a': 6} Good!

s.iloc[1] = x.iloc[:]

s.iloc[1] # => {'a': 4} What!
```

Oops.