to_datetime parses dates incorrectly when the format includes '%W' and does not include day of week plus calendar year
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

# I'm not sure how a bare week should be interpreted.  But the result should
# probably still be of the same week number
print(pd.to_datetime('20', format='%W').strftime('%W'))  # 01

# Parsing is broken even when the week is fully specified
print(pd.to_datetime('2017-20', format='%Y-%W').strftime('%Y-%W'))  # 2017-00
```
#### Problem description

to_datetime parses a week XX as 0 for any valid XX.  Interestingly, an impossible XX (ex: '70') throws a ValueError as expected.

#### Expected Output
20
2017-20

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.4.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-79-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.2
pytest: None
pip: 9.0.1
setuptools: 36.0.1
Cython: 0.24.1
numpy: 1.13.0
scipy: 0.18.1
xarray: None
IPython: 6.1.0
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.0
openpyxl: 2.4.0
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: 3.3.3
bs4: 4.2.1
html5lib: 0.999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
