Fix SEGV in TCN negotiation if no variants are acceptable.

PR: 1987
Submitted By: Martin Plechsmid <plechsmi@karlin.mff.cuni.cz>
Reviewed By: Paul Sutton <pcs@apache.org>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@82415 13f79535-47bb-0310-9956-ffa450edef68