ERR: Segfault with df.astype(category) on empty dataframe
Pandas segfaults when calling `DataFrame.astype('category')` on an empty dataframe. This fails in `0.21.0rc1`, `0.20.3`, and probably previous versions as well.

```python
In [1]: import pandas as pd

In [2]: df = pd.DataFrame({'x': ['a', 'b', 'c'], 'y': ['a', 'b', 'c'], 'z': ['a', 'b', 'c']})

In [3]: empty = df.iloc[:0].copy()  # copy is necessary, no segfault without it

In [4]: empty.astype('category')
Segmentation fault: 11
```

For non-empty frames, an error message is raised saying this operation isn't supported yet. Also note that a copy is needed to cause the segfault, without it the error message is still raised.

<details>
<summary>pd.show_versions:</summary>

```python
In [5]: pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.4.final.0
python-bits: 64
OS: Darwin
OS-release: 16.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.3
pytest: 3.1.0
pip: 9.0.1
setuptools: 36.4.0
Cython: 0.25.2
numpy: 1.13.3
scipy: 0.19.1
xarray: None
IPython: 6.1.0
sphinx: 1.5.3
patsy: None
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: 1.1.9
pymysql: None
psycopg2: None
jinja2: 2.9.5
s3fs: 0.1.1
pandas_gbq: None
pandas_datareader: None
```
</details>

