mod_session_dbd: fix a segmentation fault in the function dbd_remove.

The segmentation fault is caused by an uninitialized function pointer session_dbd_acquire_fn.
PR 53452

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1408958 13f79535-47bb-0310-9956-ffa450edef68