value_labels fails to assign value_label_dict, leads to AttributeError
#### Code Sample, a copy-pastable example if possible

```python
def test_StataReader(stata_path):
    """
    Args:
        stata_path - path to literally any valid stata file.
    """
    # Step 1: Delay reading value labels, which is done automaticaly when
    # decoding categoricals. Otherwise this only works if we have a stata file
    # with format verison <= 108
    reader = pandas.io.stata.StataReader(stata_path,
                                         convert_categoricals=False)
    # Patch format to be <= 108
    # https://github.com/pandas-dev/pandas/blob/master/pandas/io/stata.py#L1341-L1344
    with patch.object(reader, 'format_version', 108):
        # Trigger AttributeError when _read_value_labels immediately returns without
        # assigning value_label_dict, which value_labels() expects to exist
        # https://github.com/pandas-dev/pandas/blob/master/pandas/io/stata.py#L1745
        reader.value_labels()
```

#### Problem description

An unlikely code path will lead to an AttributeError.

#### Expected Output

I thin it is reasonable to expect `value_labels()` to return either `None` or `{}` when the stata file's `format_version` is <= 108

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

<details>

>>> pandas.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Darwin
OS-release: 17.3.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.21.0
pytest: 3.0.7
pip: 9.0.1
setuptools: 27.2.0
Cython: None
numpy: 1.13.3
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
