mod_ext_filter: Fix a problem building argument lists which

occasionally caused exec to fail.

The argument array passed to apr_proc_create() needs to have
a NULL entry at the end.

PR:		15491


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@98258 13f79535-47bb-0310-9956-ffa450edef68