mod_auth_digest: fix segfaults during shared memory exhaustion
The apr_rmm_addr_get/apr_rmm_malloc() combination did not correctly check for a malloc failure, leading to crashes when we ran out of the limited space provided by AuthDigestShmemSize. This patch replaces all these calls with a helper function that performs this check.
Additionally, fix a NULL-check bug during entry garbage collection. Submitted by: jchampion Reviewed/backported by: jim