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


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1773069 13f79535-47bb-0310-9956-ffa450edef68