Avoid making 0-sized series
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

giuliolunati/ren-c,rgchris/ren-c,rgchris/ren-c,hostilefork/rebol,mbk/ren-c,kealist/ren-c,giuliolunati/ren-c,rgchris/ren-c,hostilefork/rebol,codebybrett/ren-c,draegtun/ren-c,kealist/ren-c,kealist/ren-c,codebybrett/ren-c,draegtun/ren-c,codebybrett/ren-c,giuliolunati/ren-c,codebybrett/ren-c,codebybrett/ren-c,codebybrett/ren-c,hostilefork/rebol,mbk/ren-c,giuliolunati/ren-c,rgchris/ren-c,draegtun/ren-c,kealist/ren-c,kealist/ren-c,mbk/ren-c,hostilefork/rebol,hostilefork/rebol,rgchris/ren-c,mbk/ren-c,draegtun/ren-c,kealist/ren-c,giuliolunati/ren-c,hostilefork/rebol,draegtun/ren-c,draegtun/ren-c