storage: fix bucketiterator next

Fixes #1628

Change-Id: I990227a6ce15a957a7ba0b8d534617d779a0b841
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/47131
Reviewed-by: Cody Oss <codyoss@google.com>
Reviewed-by: James Hartig <fastest963@gmail.com>
diff --git a/storage/bucket.go b/storage/bucket.go
index 15cb01e..fe080bf 100644
--- a/storage/bucket.go
+++ b/storage/bucket.go
@@ -1168,9 +1168,9 @@
 	if err := it.nextFunc(); err != nil {
 		return nil, err
 	}
-	item := it.items[0]
-	it.items = it.items[1:]
-	return item, nil
+	b := it.buckets[0]
+	it.buckets = it.buckets[1:]
+	return b, nil
 }
 
 // PageInfo supports pagination. See the google.golang.org/api/iterator package for details.