Ensure full distribution description is printed in svn --version -v

* subversion/libsvn_subr/sysinfo.c
  (stringbuf_split_key): fix buffer length calculation

Approved by: breser


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1544878 13f79535-47bb-0310-9956-ffa450edef68