Reject NUL byte in URLs

[chris34]

Sentry reported f.e. for the URL
http://ubuntuusers.de/calendar/2014/03/11/ubuntu-developer-summit-14-1%00%EF%BF%BD%EF%BF%BD%EF%BF%BD%EF%BF%BD%2527%2522/ics/ the traceback

ValueError: A string literal cannot contain NUL (0x00) characters.
  File "django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "django/core/handlers/base.py", line 181, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "inyoka/portal/views.py", line 1464, in calendar_ical
    event = Event.objects.get(slug=slug)
  File "django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "django/db/models/query.py", line 431, in get
    num = len(clone)
  File "django/db/models/query.py", line 262, in __len__
    self._fetch_all()
  File "django/db/models/query.py", line 1324, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "django/db/models/query.py", line 51, in __iter__
    results = compiler.execute_sql(chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size)
  File "django/db/models/sql/compiler.py", line 1175, in execute_sql
    cursor.execute(sql, params)
  File "django/db/backends/utils.py", line 66, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "django/db/backends/utils.py", line 75, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)