mod_slotmem_shm: Fix mistaken reset of num_free for restored shm

trunk patch: https://svn.apache.org/viewvc?view=revision&revision=1430869

Submitted by: jim
Reviewed by: humbedooh, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1432601 13f79535-47bb-0310-9956-ffa450edef68