mod_slotmem_shm: follow up to r1830800, preserve callee API.

The caller might (and actually balancer_child_init() does) rely on
slotmem_attach() to not mangle the passed in/out slotmem pointer on
failure.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1831394 13f79535-47bb-0310-9956-ffa450edef68