Fix race condition after launching EC2 instance

It is possible for an instance to not immediately exist after it is
launched. Previously, InstanceStateRefreshFunc would crash if this race
condition were realized.

This change takes the exact same approach of the function above,
AMIStateRefreshFunc, treating 'InvalidInstanceID.NotFound' as if there
were an empty result.

builder/amazon/common/state.go

@@ -62,11 +62,16 @@ func InstanceStateRefreshFunc(conn *ec2.EC2, i *ec2.Instance) StateRefreshFunc {
 	return func() (interface{}, string, error) {
 		resp, err := conn.Instances([]string{i.InstanceId}, ec2.NewFilter())
 		if err != nil {
+			if ec2err, ok := err.(*ec2.Error); ok && ec2err.Code == "InvalidInstanceID.NotFound" {
+				// Set this to nil as if we didn't find anything.
+				resp = nil
+			} else {
 				log.Printf("Error on InstanceStateRefresh: %s", err)
 				return nil, "", err
 			}
+		}
 
-		if len(resp.Reservations) == 0 || len(resp.Reservations[0].Instances) == 0 {
+		if resp == nil || len(resp.Reservations) == 0 || len(resp.Reservations[0].Instances) == 0 {
 			// Sometimes AWS just has consistency issues and doesn't see
 			// our instance yet. Return an empty state.
 			return nil, "", nil