to_numeric crashes python on empty dataframe column
#### Crash sample

```python
In[2]: df = pd.DataFrame(columns=['maybe_empty_column'])
  ...: pd.to_numeric(df['maybe_empty_column'], errors='ignore')

Process finished with exit code -1073741819 (0xC0000005)

```
#### Problem description
When calling `to_numeric` on an empty DataFrame column, python crashes with an access violation exit code (0xC0000005).

#### Expected Output
Should return an empty series with default dtype for empty series (int64)

for some reason, some empty Series objects work and some don't:

```python
empty_that_works = pd.Series(['blabla',])[1:]
pd.to_numeric(empty_that_works)

empty_that_doesnt_work = pd.Series(dtype=object)
pd.to_numeric(empty_that_doesnt_work)
```

#### Output of ``pd.show_versions()``

<details>
# Paste the output here pd.show_versions() here
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Windows
OS-release: 8.1
machine: AMD64
processor: Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
pandas: 0.20.1
pytest: 3.0.7
pip: 9.0.1
setuptools: 34.3.1
Cython: None
numpy: 1.12.1
scipy: 0.18.1
xarray: None
IPython: 5.3.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: None
numexpr: 2.6.2
feather: None
matplotlib: 1.5.3
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.5.1
html5lib: 0.9999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>
