provider/aws: Fix panic on nil route configs

When creating an `aws_route_table`, if a `route` configuration block is left `nil`, Terraform would previously panic. This allows Terraform to catch a faulty interface conversion during the resource create. The resource will still fail to apply, however, since every item in the `route` element is `Optional` we cannot currently catch this error during plan time, via validation.

Fixes: #13545

builtin/providers/aws/resource_aws_route_table.go

@@ -452,7 +452,10 @@ func resourceAwsRouteTableDelete(d *schema.ResourceData, meta interface{}) error
 
 func resourceAwsRouteTableHash(v interface{}) int {
 	var buf bytes.Buffer
-	m := v.(map[string]interface{})
+	m, castOk := v.(map[string]interface{})
+	if !castOk {
+		return 0
+	}
 
 	if v, ok := m["ipv6_cidr_block"]; ok {
 		buf.WriteString(fmt.Sprintf("%s-", v.(string)))

builtin/providers/aws/resource_aws_route_table_test.go

@@ -2,6 +2,7 @@ package aws
 
 import (
 	"fmt"
+	"regexp"
 	"testing"
 
 	"github.com/aws/aws-sdk-go/aws"
@@ -183,6 +184,21 @@ func TestAccAWSRouteTable_tags(t *testing.T) {
 	})
 }
 
+func TestAccAWSRouteTable_panic(t *testing.T) {
+	resource.Test(t, resource.TestCase{
+		PreCheck:      func() { testAccPreCheck(t) },
+		IDRefreshName: "aws_route_table.foo",
+		Providers:     testAccProviders,
+		CheckDestroy:  testAccCheckRouteTableDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config:      testAccRouteTableConfigPanic,
+				ExpectError: regexp.MustCompile("The request must contain the parameter destinationCidrBlock or destinationIpv6CidrBlock"),
+			},
+		},
+	})
+}
+
 func testAccCheckRouteTableDestroy(s *terraform.State) error {
 	conn := testAccProvider.Meta().(*AWSClient).ec2conn
 
@@ -497,3 +513,16 @@ resource "aws_route_table" "foo" {
 	propagating_vgws = ["${aws_vpn_gateway.foo.id}"]
 }
 `
+
+const testAccRouteTableConfigPanic = `
+resource "aws_vpc" "foo" {
+	cidr_block = "10.2.0.0/16"
+}
+
+resource "aws_route_table" "foo" {
+	vpc_id = "${aws_vpc.foo.id}"
+
+  route {
+  }
+}
+`