PoE plugin crashes inventory job if type of netbox is unknown

[lunkwill42]

This line causes the inventory job to crash for a device that has no know type yet (as may happen on the first collection run):

https://github.com/UNINETT/nav/blob/9288d2f87d6993b1ea21616ac731955e8b3a99ea/python/nav/ipdevpoll/plugins/poe.py#L38

From the log

2017-11-15 15:45:50,806 [ERROR jobs.jobhandler] [statuscheck ] Plugin nav.ipdevpoll.plugins.poe.Poe(u'') reported an unhandled failure
Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/twisted/internet/defer.py", line 653, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/source/python/nav/ipdevpoll/jobs.py", line 222, in next_plugin
    df = defer.maybeDeferred(plugin_instance.handle)
  File "/usr/local/lib/python2.7/dist-packages/twisted/internet/defer.py", line 150, in maybeDeferred
    result = f(*args, **kw)
  File "/usr/local/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1532, in unwindGenerator
    return _inlineCallbacks(None, gen, Deferred())
--- <exception caught here> ---
  File "/usr/local/lib/python2.7/dist-packages/twisted/internet/defer.py", line 1386, in _inlineCallbacks
    result = g.send(result)
  File "/source/python/nav/ipdevpoll/plugins/poe.py", line 38, in handle
    if self.netbox.type.vendor.id == 'cisco':
exceptions.AttributeError: 'NoneType' object has no attribute 'vendor'

This is on the master branch, not on a release yet