Avoid uninitialized var

Complained by coverity
Avoid making 0-sized series

This fixes a regression caused by:

commit 7d090b22250dd9f77521f5cf7d0cfaa75b710742
Author: Shixin Zeng <szeng@atronixengineering.com>
Date:   Mon Sep 15 12:37:14 2014 -0400

    Avoid undefined behavior caused by malloc(0)

    Found by clang-analyzer

Bug reported by kealist@stackoverflow