data after null character dropped in `read_csv`
#### Code Sample

```python
In [1]: from pandas.compat import StringIO
   ...: from pandas import read_csv
   ...: data = 'x,y\ntest,Reg\n\x00\x00\x00,Reg\nI,Swp\nI,Swp\n'
   ...: read_csv(StringIO(data), engine='c')
   ...:
Out[1]:
      x    y
0  test  Reg
1   NaN  Reg
2     I  Swp
3     I  Swp

In [2]: data = 'x,y\n\x00,Reg\n\x00\x00\x00,Reg\nI,Swp\nI,Swp\n'
   ...: read_csv(StringIO(data), engine='c')
   ...:
Out[2]:
     x    y
0  NaN  Reg
1  NaN  Reg
2    I  Swp
3    I  Swp

In [4]: data = 'x,y\ntest\x00test,Reg\n\x00\x00\x00,Reg\nI,Swp\nI,Swp\n'
   ...: read_csv(StringIO(data), engine='c')
   ...:
Out[4]:
      x    y
0  test  Reg
1   NaN  Reg
2     I  Swp
3     I  Swp
```

#### Problem description

Within a single field data before a `NUL` character is kept, but everything after, until the delimiter, is dropped. This result seems counterintuitive to me. I'd either expect for the field to

1) always become NaN with the other data dropped or
2) expect to have a string with the data and `NUL` characters (or even dropped would probably be fine). NaN only If the field was exclusively `NUL` (see expected output section below)

Might be related to #2741 or the related fix.

#### Expected Output
```python
# last case
In [X]: data = 'x,y\ntest\x00test,Reg\n\x00\x00\x00,Reg\nI,Swp\nI,Swp\n'
   ...: read_csv(StringIO(data), engine='c')
   ...:
Out[X]:
      x    y
0  test\00test  Reg
1   NaN  Reg
2     I  Swp
3     I  Swp

# another case
In [X]: data = 'x,y\n\x00test,Reg\n\x00\x00\x00,Reg\nI,Swp\nI,Swp\n'
   ...: read_csv(StringIO(data), engine='c')
   ...:
Out[X]:
      x    y
0  \00test  Reg
1   NaN  Reg
2     I  Swp
3     I  Swp
```


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

<details>

In [9]: import pandas as pd
   ...: pd.show_versions()
   ...:

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Darwin
OS-release: 15.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.21.0
pytest: 3.1.2
pip: 9.0.1
setuptools: 35.0.2
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
pyarrow: 0.8.0
xarray: None
IPython: 6.1.0
sphinx: 1.6.2
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.5.0a1
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.8.0
bs4: 4.5.3
html5lib: 0.999
sqlalchemy: 1.1.5
pymysql: None
psycopg2: None
jinja2: 2.9.5
s3fs: 0.1.0
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
