provider/aws: Guard against panic when no aws_default_vpc found (#15070)
provider/aws: Guard against panic when no aws_default_vpc found (#15070)

Found as part of #15065, when there is no default VPC, Terraform will
throw a panic. This prevents that as a user should never get that

```
% make testacc TEST=./builtin/providers/aws/ TESTARGS='-run=TestAccAWSDefaultVpc_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/06/05 12:16:09 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws/ -v -run=TestAccAWSDefaultVpc_basic -timeout 120m
=== RUN   TestAccAWSDefaultVpc_basic
--- PASS: TestAccAWSDefaultVpc_basic (44.65s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	44.669s
```
charles-at-geospock/terraform-provider-aws,ClementCunin/terraform-provider-aws,ClementCunin/terraform-provider-aws,ClementCunin/terraform-provider-aws,nbaztec/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,Ninir/terraform-provider-aws,ashinohara/terraform-provider-aws,nbaztec/terraform-provider-aws,ashinohara/terraform-provider-aws,kjmkznr/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,terraform-providers/terraform-provider-aws,nbaztec/terraform-provider-aws,Ninir/terraform-provider-aws,nbaztec/terraform-provider-aws,terraform-providers/terraform-provider-aws,terraform-providers/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,charles-at-geospock/terraform-provider-aws,kjmkznr/terraform-provider-aws,kjmkznr/terraform-provider-aws,Ninir/terraform-provider-aws,ClementCunin/terraform-provider-aws,ashinohara/terraform-provider-aws,terraform-providers/terraform-provider-aws,ashinohara/terraform-provider-aws