Fix nil-deref panic from Put.

service/datastore/datastore.go

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package datastore
 
 import (
         "fmt"
         "reflect"
 
@@ skipping 290 matching lines @@
         mat := parseMultiArg(slice.Type())
 
         keys, vals, err := mat.GetKeysPMs(d.aid, d.ns, slice, false)
         if err != nil {
                 return err
         }
 
         lme := errors.NewLazyMultiError(len(keys))
         i := 0
         err = d.RawInterface.PutMulti(keys, vals, func(key *Key, err error) error {
-                if key != keys[i] {
+                if !lme.Assign(i, err) && key != keys[i] {
                         mat.setKey(slice.Index(i), key)
                 }
-                lme.Assign(i, err)
                 i++
                 return nil
         })
 
         if err == nil {
                 err = lme.Get()
         }
         return err
 }
 
 func (d *datastoreImpl) DeleteMulti(keys []*Key) (err error) {
         lme := errors.NewLazyMultiError(len(keys))
         i := 0
         extErr := d.RawInterface.DeleteMulti(keys, func(internalErr error) error {
                 lme.Assign(i, internalErr)
                 i++
                 return nil
         })
         err = lme.Get()
         if err == nil {
                 err = extErr
         }
         return
 }
 
 func (d *datastoreImpl) Raw() RawInterface {
         return d.RawInterface
 }