provider/aws: Guard against panic in aws_vpc_endpoint_association (ha…

…shicorp#11613)

I believe that if no VPC Endpoints were returned from the AWS API, we
were not guarding against a panic. We were strill trying to inspect the
RouteTableIds. This commit will ensure that no errors are thrown before
trying to use the RouteTableIds

```
% make testacc TEST=./builtin/providers/aws TESTARGS='-run=TestAccAWSVpcEndpointRouteTableAssociation_'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/02/01 18:06:29 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSVpcEndpointRouteTableAssociation_ -timeout 120m
=== RUN   TestAccAWSVpcEndpointRouteTableAssociation_basic
--- PASS: TestAccAWSVpcEndpointRouteTableAssociation_basic (42.83s)
PASS
ok  	github.com/hashicorp/terraform/builtin/providers/aws	42.859s
```

builtin/providers/aws/resource_aws_vpc_endpoint_route_table_association.go

@@ -72,9 +72,13 @@ func resourceAwsVPCEndpointRouteTableAssociationRead(d *schema.ResourceData, met
 	rtId := d.Get("route_table_id").(string)
 
 	vpce, err := findResourceVPCEndpoint(conn, endpointId)
-	if err, ok := err.(awserr.Error); ok && err.Code() == "InvalidVpcEndpointId.NotFound" {
-		d.SetId("")
-		return nil
+	if err != nil {
+		if err, ok := err.(awserr.Error); ok && err.Code() == "InvalidVpcEndpointId.NotFound" {
+			d.SetId("")
+			return nil
+		}
+
+		return err
 	}
 
 	found := false
@@ -143,6 +147,10 @@ func findResourceVPCEndpoint(conn *ec2.EC2, id string) (*ec2.VpcEndpoint, error)
 		return nil, err
 	}
 
+	if output.VpcEndpoints == nil {
+		return nil, fmt.Errorf("No VPC Endpoints were found for %q", id)
+	}
+
 	return output.VpcEndpoints[0], nil
 }