[bugfix] Don't save instance ID if association fails

- typically happens when no internet GW is available in VPC

builtin/providers/aws/resource_aws_eip.go

@@ -140,7 +140,9 @@ func resourceAwsEipRead(d *schema.ResourceData, meta interface{}) error {
 	address := describeAddresses.Addresses[0]
 
 	d.Set("association_id", address.AssociationID)
+	if address.InstanceID != nil {
 		d.Set("instance", address.InstanceID)
+	}
 	d.Set("private_ip", address.PrivateIPAddress)
 	d.Set("public_ip", address.PublicIP)
 
@@ -173,6 +175,9 @@ func resourceAwsEipUpdate(d *schema.ResourceData, meta interface{}) error {
 		log.Printf("[DEBUG] EIP associate configuration: %#v (domain: %v)", assocOpts, domain)
 		_, err := ec2conn.AssociateAddress(assocOpts)
 		if err != nil {
+			// Prevent saving instance if association failed
+			// e.g. missing internet gateway in VPC
+			d.Set("instance", "")
 			return fmt.Errorf("Failure associating instances: %s", err)
 		}
 	}