Topology map generation breaks on circuits with no Z termination interface set

[miek]

Issue type: Bug report

Python version: 2.7.13
NetBox version: 2.0.6

I had a circuit with A and Z terminations defined, but no termination device/interface set on the Z side. There is a topology map defined with the device pattern .* to add everything for testing.

When trying to render the graph I get the following error:

There was an error generating the requested graph. Ensure that the GraphViz executables have been installed correctly.

I believe this is caused by https://github.com/digitalocean/netbox/blob/293dbd8a8b2c291d1ae9efc7b2f2137fc896833b/netbox/extras/models.py#L374 where peer_termination is checked, but peer_termination.interface is not. The following exception is then raised (triggering the error above):

<type 'exceptions.AttributeError'>
'NoneType' object has no attribute 'device'