Alert profiles with vendor-based filters seems to crash the Alertengine completely

[lunkwill42]

By creating an alert profile subscribing to a filter group containing a filter based on vendor ("equipment vendor"), it seems it is possible to provoke the alertengine to crash and stop. This traceback is logged:

Traceback (most recent call last):
  File "/usr/lib/nav/alertengine.py", line 153, in main
    check_alerts(debug=opttest)
  File "/usr/lib/python2.7/dist-packages/nav/alertengine/base.py", line 63, in check_alerts
    handle_new_alerts(new_alerts)
  File "/usr/lib/python2.7/dist-packages/django/db/transaction.py", line 394, in inner
    return func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/nav/alertengine/base.py", line 187, in handle_new_alerts
    permissions)
  File "/usr/lib/python2.7/dist-packages/nav/alertengine/base.py", line 201, in _check_match_and_permission
    if memoized_check_alert(alert, filtergroupcontents, 'match check'):
  File "/usr/lib/python2.7/dist-packages/django/utils/functional.py", line 38, in wrapper
    result = func(*args)
  File "/usr/lib/python2.7/dist-packages/nav/alertengine/base.py", line 459, in check_alert_against_filtergroupcontents
    matches = content.filter.check(alert) != content.positive
  File "/usr/lib/python2.7/dist-packages/nav/models/profiles.py", line 861, in check
    if AlertQueue.objects.filter(**filtr).exclude(**exclude).extra(
  File "/usr/lib/python2.7/dist-packages/django/db/models/manager.py", line 92, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/db/models/query.py", line 691, in filter
    return self._filter_or_exclude(False, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/django/db/models/query.py", line 709, in _filter_or_exclude
    clone.query.add_q(Q(*args, **kwargs))
  File "/usr/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1331, in add_q
    clause, require_inner = self._add_q(where_part, self.used_aliases)
  File "/usr/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1358, in _add_q
    current_negated=current_negated, connector=connector)
  File "/usr/lib/python2.7/dist-packages/django/db/models/sql/query.py", line 1225, in build_filter
    lookups, value)
  File "/usr/lib/python2.7/dist-packages/django/db/models/fields/related.py", line 1530, in get_lookup_constraint
    raise exceptions.FieldError('Relation fields do not support nested lookups')
FieldError: Relation fields do not support nested lookups