holtWintersForecast fails with future dates.

[thomaslitton]

I'm using the holtWintersForecast function with a small amount of data (< 24 hours). If i set the until date 2 weeks ahead of the from date, i get the error below.

Here are the combinations of from and until i was able to reproduce this error:

&from=-15d&until=-1h
&from=-1w&until=20171027 (1 week + 1 day in the future)
&until=20171102 (2 weeks in the future)

If the from and until date were closer on any of the above by 1 day, then it works fine.

Traceback (most recent call last):
File "/srv/graphite-pypy/site-packages/flask/app.py", line 1982, in wsgi_app
response = self.full_dispatch_request()
File "/srv/graphite-pypy/site-packages/flask/app.py", line 1614, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/srv/graphite-pypy/site-packages/flask/app.py", line 1517, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "/srv/graphite-pypy/site-packages/flask/app.py", line 1612, in full_dispatch_request
rv = self.dispatch_request()
File "/srv/graphite-pypy/site-packages/flask/app.py", line 1598, in dispatch_request
return self.view_functionsrule.endpoint
File "/srv/graphite-pypy/site-packages/graphite_api/app.py", line 401, in render
series_list = evaluateTarget(context, target, data_store)
File "/srv/graphite-pypy/site-packages/graphite_api/app.py", line 496, in evaluateTarget
result = evaluateTokens(requestContext, tokens, data_store)
File "/srv/graphite-pypy/site-packages/graphite_api/app.py", line 506, in evaluateTokens
return evaluateTokens(requestContext, tokens.expression, data_store)
File "/srv/graphite-pypy/site-packages/graphite_api/app.py", line 520, in evaluateTokens
ret = func(requestContext, *args, **kwargs)
File "/srv/graphite-pypy/site-packages/graphite_api/functions.py", line 2261, in holtWintersForecast
analysis = holtWintersAnalysis(bootstrap)
File "/srv/graphite-pypy/site-packages/graphite_api/functions.py", line 2216, in holtWintersAnalysis
next_last_seasonal = getLastSeasonal(i+1)
File "/srv/graphite-pypy/site-packages/graphite_api/functions.py", line 2177, in getLastSeasonal
return seasonals[j]
IndexError: list index out of range

this is similar to: #211