Fix internal handling of registered cleanups in alloc.c by making sure that

NULL-specified callback functions (we don't do this in the official set of
modules, but third-party modules do) for ap_register_cleanup() always mean the
dummy ap_null_cleanup() so we don't dump core later when running the cleanups.

Submitted by: J.Kean Johnston <jkj@sco.com>
Reviewed by: Ralf S. Engelschall
PR: 3307, 3250, 3256


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@82354 13f79535-47bb-0310-9956-ffa450edef68