resource/aws_instance: Guard check for aws_instance UserData to preve…

…nt panic

Fixes: hashicorp#581

This is just a simple guard clause to make sure that we are checking
UserData is not nil before we check that the UserData.Value is not nil

aws/resource_aws_instance.go

@@ -718,7 +718,7 @@ func resourceAwsInstanceRead(d *schema.ResourceData, meta interface{}) error {
 		if err != nil {
 			return err
 		}
-		if attr.UserData.Value != nil {
+		if attr.UserData != nil && attr.UserData.Value != nil {
 			d.Set("user_data", userDataHashSum(*attr.UserData.Value))
 		}
 	}