ap_rgetline_core: insure that the output string is null terminated

when exiting with APR_ENOSPC

Submitted by: Tsurutani Naoki <turutani scphys.kyoto-u.ac.jp>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@103482 13f79535-47bb-0310-9956-ffa450edef68