mod_ext_filter: don't segfault if content-type is not set.

PR:                        10617
Submitted by:              Arthur P. Smith <apsmith@aps.org>
Reviewed/modified by:      Jeff Trawick


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@96003 13f79535-47bb-0310-9956-ffa450edef68