If the mmap cache cannot be initialized (such as insufficient
permissions or SELinux/AppArmor denial), we are supposed to fall back to
our 1.8 behavior of LDB cache only. However, we weren't properly
checking that the cache had been set up and we were always attempting to
dereference the mmap context in fill_pwent() and fill_grent().
Fixes
https://fedorahosted.org/sssd/ticket/1346