Buffer too small error on newick output

If we run this command in msprime:
```
$ python3 mspms_dev.py 15 1 -r 0 1.0 -eG 1.0 5.25 -eG 2.0 10 -G 4 -eN 3.0 1.0 -T
```
we currently get a 
```
_tskit.LibraryError: Supplied buffer is too small
```
So, our calculation of a good buffer size is out somehow. Should be fixed before 0.3.0 is shipped.