[dataformats] Invalid data format may generate 500 error on "fork"

Upon forking a dataformat, the user may slightly modify the said format and generate an internal logical error leading to a 500 page displayed.

I just received this e-mail from our beat.web server:

Internal Server Error: /platform/dataformats/fork/chichan/a-collection-of-linear_machines/1/
Traceback (most recent call last):
  File "/remote/sw/beat.env.web-0.0.1/usr/lib/python2.7/site-packages/django/core/handlers/base.py", line 132, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/remote/sw/beat.env.web-0.0.1/usr/lib/python2.7/site-packages/django/utils/decorators.py", line 110, in _wrapped_view
    response = view_func(request, *args, **kwargs)
  File "/remote/sw/beat.env.web-0.0.1/usr/lib/python2.7/site-packages/django/contrib/auth/decorators.py", line 22, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "/remote/sw/beat.web/src/beat.web/beat/web/dataformats/views.py", line 165, in fork_dataformat
    dataformat.author.username,
AttributeError: 'NoneType' object has no attribute 'author'

Indeed, this bit of code needs protection:

if form.is_valid():
            (dataformat, errors) = DataFormat.objects.create_dataformat(
                author=request.user,
                name=form.cleaned_data['name'],
                short_description=form.cleaned_data['short_description'],
                description=form.cleaned_data['description'],
                declaration=form.cleaned_data['file'],
                fork_of=fork_of
            )

            # Redirect to the page of the dataformat just created
            return HttpResponseRedirect(
                    reverse('dataformats-view',
                        args=(
                            dataformat.author.username,
                            dataformat.name,
                            dataformat.version,
                        ),
                    )
                )

For example, it does not handle the case in which the "validation" generates an error and the returned data format is None, generating the error above. This code must be corrected both on the branch "0.14" as well as on the "master" branch.