Potential bug with read_fwf
Hi,

I may have encountered a bug with read_fwf. From my files, some digits of numbers with more than 2 digits before the decimal turn to be missing, but only sometimes. I use two files to show the potential bug: [test1.txt](https://github.com/pandas-dev/pandas/files/1643703/test1.txt) and [test2.txt](https://github.com/pandas-dev/pandas/files/1643702/test2.txt)


```python
In [1] : pd_test1 = pd.read_fwf('test1.txt', header= None)     
         pd_test1[5][0]
Out [1] : 101.197524

In [2] : pd_test2 = pd.read_fwf('test2.txt', header= None)    

In [3] : pd_test2[5][223]
Out [3] : 1.197524 # Should be 101.197524 (note: the row 1 in "test1.txt" is a copy-paste of the row 224 in "test2.txt")

In [4] : pd_test2[5][274]
Out [4] : 9.5568229999999996 # Should be 109.556823

In [5] : pd_test2[5][369]
Out [4] : 24.066264 # Should be 124.066264
```
#### Problem description

The problem is perfectly exemplified from the output [3], which should be the same as the output [1], as the first row in "test1.txt" is simply a copy-paste of the 224th row in "test2.txt". Though, I noticed that if I add a space before, for instance, "101.197524" in "test2.txt" (line 224), the output becomes correct. 

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

</details>
INSTALLED VERSIONS
------------------

commit: None
python: 2.7.12.final.0
python-bits: 64
OS: Linux
OS-release: 4.13.0-26-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: fr_FR.UTF-8
LOCALE: None.None

pandas: 0.22.0
pytest: 3.2.5
pip: 9.0.1
setuptools: 20.7.0
Cython: None
numpy: 1.14.0
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 5.5.0
sphinx: None
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0b10
sqlalchemy: None
pymysql: 0.7.2.None
psycopg2: None
jinja2: 2.8
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
