date_range segfaults on None utcoffset
As reported at pendulum, there are cases where creating a ```date_range``` fails with a segfault: [pendulum issues 131](https://github.com/sdispater/pendulum/issues/131).

#### Code Sample

```python
from datetime import datetime, timedelta

from pendulum.tz.timezone import UTCTimezone
from pendulum.tz.timezone_info import UTC
import pandas as pd

tomorrow = datetime.now() + timedelta(hours=24)
now = datetime.now()

r = pd.date_range(now, tomorrow, freq='1H')

r.tz = UTC
# prints out the current date and time
print(r[0])

r.tz = UTCTimezone
# seg faults
print(r[0])
```

#### Problem description
As I understand it is allowed for ```tzinfo``` objects to return ```None``` for ```utcoffset``` according to the [Python docs](https://docs.python.org/3.6/library/datetime.html#datetime.tzinfo.utcoffset). However, in [_get_dst_info](https://github.com/pandas-dev/pandas/blob/c8dafb5a7ae9fe42b9d15c47082a6fb139e78b5d/pandas/_libs/tslib.pyx#L4332) of ```tslib``` it is assumed _not_ to return ```None```. As I understand from the aforementioned ```pendulum``` ticket, it is correct to return ```None``` in this case.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.11.3-200.fc25.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.20.3
pytest: 3.1.3
pip: 9.0.1
setuptools: 25.1.1
Cython: None
numpy: 1.13.1
scipy: 0.19.1
xarray: None
IPython: 6.1.0
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.6.0
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>