Prefix import does not include VLAN

[royvegard]

Issue type: Bug report

Python version: 2.7.12
NetBox version: 1.9.7-dev 861c8b2

Importing a prefix with a valid VLAN does not include the VLAN.

Steps to reproduce

1. Create a VLAN with and ID of 100.
2. Enter the following into the Prefix import form and submit:
   192.0.2.0/24,,,,,100,Active,,False,

The prefix will be imported but it will not include the VLAN.

I believe this was introduced in d3b16ba. Line 239 is always executed - setting self.instance.vlan = vlan which sets it to None. I'm guessing that it should only be executed if exeption VLAN.MultipleObjectsReturned happens.

[jeremystretch]

Thanks!