fix error reporting

weed/storage/idx/walk.go

@@ -14,6 +14,9 @@ func WalkIndexFile(r io.ReaderAt, fn func(key types.NeedleId, offset types.Offse
 	var readerOffset int64
 	bytes := make([]byte, types.NeedleMapEntrySize*RowsToRead)
 	count, e := r.ReadAt(bytes, readerOffset)
+	if count == 0 && e == io.EOF {
+		return nil
+	}
 	glog.V(3).Infof("readerOffset %d count %d err: %v", readerOffset, count, e)
 	readerOffset += int64(count)
 	var (