Check for nil VpcId and SubnetId before use

[codekoala]

Fixes #1250

Problem

A couple of nil pointer dereference errors appeared for me while enumerating resources across multiple AWS accounts.

Solution

Include a simple check that inst.VpcId and inst.SubnetId are not nil prior to adding them as a link.

Changes Made

• check that inst.VpcId is not nil prior to using it in a models.Link
• check that inst.SubnetId is not nil prior to using it in a models.Link

How to Test

This error appeared very shortly after running komiser start. As described in #1250, I have two AWS accounts configured. This may or may not be an important piece of information for this bug.

Notes

I do not know why the EC2 instance resource came back with nil values for VPC ID and Subnet ID. This behavior only occurred after I had deleted a few EC2 instances from one AWS account. However, when I removed the cloud account from the config, the error continued to display every time I ran komiser start. That suggests to me that the offending EC2 instance belongs in the other AWS account, where I did not delete any EC2 instances. This could very well just be a red herring, but I figured I'd mention it.

Checklist

• Code follows the contributing guidelines
• Changes have been thoroughly tested
• Documentation has been updated, if necessary
• Any dependencies have been added to the project, if necessary

Reviewers