resource/aws_ec2_transit_gateway_route: Prevent crash with externally…

… removed attachment

Reference: #7111

Previous output from acceptance testing:

```
=== CONT  TestAccAWSEc2TransitGatewayRoute_disappears_TransitGatewayAttachment
panic: runtime error: index out of range

goroutine 1025 [running]:
github.com/terraform-providers/terraform-provider-aws/aws.resourceAwsEc2TransitGatewayRouteRead(0xc0007f84d0, 0x42c6e00, 0xc000904700, 0xc0007f84d0, 0x0)
	/Users/bflad/go/src/github.com/terraform-providers/terraform-provider-aws/aws/resource_aws_ec2_transit_gateway_route.go:128 +0x8e6
github.com/terraform-providers/terraform-provider-aws/vendor/github.com/hashicorp/terraform/helper/schema.(*Resource).Refresh(0xc0004f8b60, 0xc0009630e0, 0x42c6e00, 0xc000904700, 0xc000547cb0, 0xc00071cf01, 0xc00074c550)
	/Users/bflad/go/src/github.com/terraform-providers/terraform-provider-aws/vendor/github.com/hashicorp/terraform/helper/schema/resource.go:352 +0x160
```

Output from acceptance testing:

```
--- PASS: TestAccAWSEc2TransitGatewayRoute_disappears (310.87s)
--- PASS: TestAccAWSEc2TransitGatewayRoute_basic (314.08s)
--- PASS: TestAccAWSEc2TransitGatewayRoute_disappears_TransitGatewayAttachment (336.06s)
```

aws/resource_aws_ec2_transit_gateway_route.go

@@ -124,7 +124,7 @@ func resourceAwsEc2TransitGatewayRouteRead(d *schema.ResourceData, meta interfac
 	d.Set("destination_cidr_block", transitGatewayRoute.DestinationCidrBlock)
 
 	d.Set("transit_gateway_attachment_id", "")
-	if len(transitGatewayRoute.TransitGatewayAttachments) == 0 || transitGatewayRoute.TransitGatewayAttachments[0] != nil {
+	if len(transitGatewayRoute.TransitGatewayAttachments) > 0 && transitGatewayRoute.TransitGatewayAttachments[0] != nil {
 		d.Set("transit_gateway_attachment_id", transitGatewayRoute.TransitGatewayAttachments[0].TransitGatewayAttachmentId)
 	}
 

aws/resource_aws_ec2_transit_gateway_route_test.go

@@ -62,6 +62,33 @@ func TestAccAWSEc2TransitGatewayRoute_disappears(t *testing.T) {
 	})
 }
 
+func TestAccAWSEc2TransitGatewayRoute_disappears_TransitGatewayAttachment(t *testing.T) {
+	var transitGateway1 ec2.TransitGateway
+	var transitGatewayRoute1 ec2.TransitGatewayRoute
+	var transitGatewayVpcAttachment1 ec2.TransitGatewayVpcAttachment
+	resourceName := "aws_ec2_transit_gateway_route.test"
+	transitGatewayVpcAttachmentResourceName := "aws_ec2_transit_gateway_vpc_attachment.test"
+	transitGatewayResourceName := "aws_ec2_transit_gateway.test"
+
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t); testAccPreCheckAWSEc2TransitGateway(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckAWSEc2TransitGatewayRouteDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccAWSEc2TransitGatewayRouteConfigDestinationCidrBlock(),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckAWSEc2TransitGatewayExists(transitGatewayResourceName, &transitGateway1),
+					testAccCheckAWSEc2TransitGatewayRouteExists(resourceName, &transitGatewayRoute1),
+					testAccCheckAWSEc2TransitGatewayVpcAttachmentExists(transitGatewayVpcAttachmentResourceName, &transitGatewayVpcAttachment1),
+					testAccCheckAWSEc2TransitGatewayVpcAttachmentDisappears(&transitGatewayVpcAttachment1),
+				),
+				ExpectNonEmptyPlan: true,
+			},
+		},
+	})
+}
+
 func testAccCheckAWSEc2TransitGatewayRouteExists(resourceName string, transitGatewayRoute *ec2.TransitGatewayRoute) resource.TestCheckFunc {
 	return func(s *terraform.State) error {
 		rs, ok := s.RootModule().Resources[resourceName]