read_csv: Inconsistent behaviour with parse_dates if the first row is short
I suspect this is related to #6710.

```
import pandas
from io import StringIO
t = "a,b,c,d\n1970-01-01,2,3,4"
s2 = "a,b,c\n1970-01-01,2,3\n1980-01-01,2,3,4"
s = "a,b,c\n1970-01-01,2,3,4"
pandas.read_csv(StringIO(t), parse_dates=['a'])
pandas.read_csv(StringIO(s2), parse_dates=['a'])
pandas.read_csv(StringIO(s), parse_dates=['a'])
```

Calling `read_csv` with `parse_dates=['a']` does the correct thing for t, raises a CParserError for s2, and raises `TypeError:  'NoneType' object has no attribute '__getitem__'` for s. 

I would expect a CParserError for s as well.

Pandas version: 0.16.2
