fix segv in mod_mime if no AddTypes are configured

PR:
Obtained from:
Submitted by:	John Sterling <sterling@covalent.net>
Reviewed by:	dougm


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@90506 13f79535-47bb-0310-9956-ffa450edef68