Make the stringbuf_from_aprfile() quickpath actually kick in.

* subversion/libsvn_subr/io.c
  (stringbuf_from_aprfile): APR does not set the name but the fname field

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1544715 13f79535-47bb-0310-9956-ffa450edef68