Fix mod_include potential segfault checking backref from unmatched regexp

http://markmail.org/message/jlc7t5edsjujbe37
Patch by rpluem, lars, niq


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@795445 13f79535-47bb-0310-9956-ffa450edef68