provider/digitalocean: Check for nil response

This applies the same fix to `digitalocean_ssh_key` as #5588 applies to
droplets. Fixes #5402. The report there gives weight to my theory that
this occurs when there are transport issues.

builtin/providers/digitalocean/resource_digitalocean_ssh_key.go

@@ -74,7 +74,7 @@ func resourceDigitalOceanSSHKeyRead(d *schema.ResourceData, meta interface{}) er
 	if err != nil {
 		// If the key is somehow already destroyed, mark as
 		// successfully gone
-		if resp.StatusCode == 404 {
+		if resp != nil && resp.StatusCode == 404 {
 			d.SetId("")
 			return nil
 		}