Fix reading beyond string in yaz_sort_spec
Fix reading beyond string in yaz_sort_spec

Spotted by Debians buildds output:
https://buildd.debian.org/fetch.cgi?pkg=yaz&arch=mips&
 ver=4.2.30-1&stamp=1334563202&file=log

dcrossleyau/yaz,nla/yaz,dcrossleyau/yaz,nla/yaz,dcrossleyau/yaz,nla/yaz,nla/yaz