Crash in tslib with certain timezone objects
#### Code Sample, a copy-pastable example if possible

```python
import win32timezone
import pywintypes
import pandas

d = pywintypes.TimeType(2001, 1, 1, tzinfo=win32timezone.TimeZoneInfo('Eastern Standard Time'))

ix = pandas.Index([d])  ## crash occurs here
```
#### Problem description

In the example above, tzinfo.utcoffset(None) returns None. In tslib.pyx _get_dst_info calls int(total_seconds(_get_utcoffset(tz, None))), and _get_utcoffset(tz, None) returns None. This causes a hard crash because total_seconds from datetime_helper.h expects a timedelta object, and crashes if the attributes "microseconds", "seconds" or "days" don't exist.

```
// from datetime_helper.h
npy_int64 get_long_attr(PyObject *o, const char *attr) {
  npy_int64 long_val;
  PyObject *value = PyObject_GetAttrString(o, attr);  <-- This returns NULL
```

When using a timezone from pytz the _utcoffset attribute is set, which is used by _get_utcoffset, and so this crash doesn't happen with that library.

#### Expected Output

It shouldn't crash. Ideally it would handle these timezone types, but at least it should raise an Exception if the timezone type isn't currently supported.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.20.0rc1+6.g844013b20.dirty
pytest: 3.0.5
pip: 9.0.1
setuptools: 34.3.3
Cython: 0.25.2
numpy: 1.11.2
scipy: None
xarray: None
IPython: 5.3.0
sphinx: 1.5.3
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: 2.4.1
xlrd: None
xlwt: None
xlsxwriter: 0.9.3
lxml: None
bs4: None
html5lib: None
sqlalchemy: 1.1.4
pymysql: None
psycopg2: 2.7.1 (dt dec pq3 ext lo64)
jinja2: 2.9.5
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>
