Error when joining string columns

```python
from datatable import *
X = Frame(A=['Ahoy yah matey!', 'hey'])
J = Frame(A=['hey'], B=['nope'])
J.key = 'A'
R = X[:, :, join(J)]
R.internal.check()
```
produces
```
AssertionError: Size of string data section in Column 1 does not correspond to the magnitude of the final offset: size = 33154, expected 400
```