BUG: Fix Segfault with pd.tslib.get_period_field (GH4519, GH4520)

[prossahl]

Fixes the following problem (issues #4519, #4520):

>>> import pandas as pd
>>> pd.tslib.get_period_field(11, 1, 1)
Exception ValueError: ValueError('Unrecognized code: 11',) in 'pandas.tslib._get_accessor_func' ignored
Segmentation fault

[jreback]

pls move this to the end of the Bugs section (as the last entry)

[prossahl]

Indeed, that would work but it seems to be simpler and cheaper to check f against NULL rather than using Cython's exception specification machinery.

[jreback]

its fine...just pointing out another way...pls rebase and squash commits into 1 and i'll merge you in

[prossahl]

Done! Thanks.

[jreback]

@prossahl thank you!