Ignore nil entries in route tables
Ignore nil entries in route tables

- possible bug in AWS SDK
  https://github.com/aws/aws-sdk-ruby/issues/1510
- class affected: Aws::EC2::RouteTable
- appears with IPv6 subnets

fixes:
```
NoMethodError:
 undefined method `destination_cidr_block' for nil:NilClass
```

cloudfoundry-incubator/bosh-aws-cpi-release,cloudfoundry-incubator/bosh-aws-cpi-release,cloudfoundry-incubator/bosh-aws-cpi-release