Fix 404 page crash

[zorun]

The 404 page crashes when the user is logged in:

  File "/home/zorun/code/ihatemoney/ihatemoney/templates/404.html", line 1, in top-level template code
    {% extends "layout.html" %}
  File "/home/zorun/code/ihatemoney/ihatemoney/templates/layout.html", line 124, in top-level template code
    {{ g.logout_form.hidden_tag() }}
  File "/home/zorun/venv/py3-ihatemoney/lib/python3.9/site-packages/jinja2/environment.py", line 474, in getattr
    return getattr(obj, attribute)
jinja2.exceptions.UndefinedError: 'flask.ctx._AppCtxGlobals object' has no attribute 'logout_form'

This is because the logout form is defined by a URL processor, and this does not seem to apply for all pages.

To solve this, simply skip the logout form if it's not defined.

[almet]

Thanks!