mod_auth_digest: avoid crash if shm init fails.

trunk patch: https://svn.apache.org/r1463047
Submitted by: sf
Reviewed by: humbedooh, covener


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1476676 13f79535-47bb-0310-9956-ffa450edef68