Direct access to signin fails, due to new is_trusted_url logic

[benoitpointet]

In get_next_url the fallback return value is the referrer, through is_trusted_url.
Yet in the case of a direct access to signin, there's no referrer and thus the signin page returns an error.

gae-init/main/util.py

Line 60 in 70d7b3c

return is_trusted_url(flask.request.referrer)

I changed that line in my project to the following (although I'm really not sure that's the right way):

  if flask.request.referrer:
    return is_trusted_url(flask.request.referrer)
  return ''