Bug 710377 - Import with chain-on-update crashes ns-slapd
    
    If you perform an import using ldif2db for a suffix that is configured
    to chain-on-update, ns-slapd will crash due to a NULL pointer dereference.
    The problem is that the chaining backends are not loaded when ns-slapd is
    run in ldif2db mode.  This results in the chaining backend being NULL.
    
    To fix this issue, we should return the local backend in the chain-on-update
    distribution function when we are unable to find a backend by name.  We can
    assume that this only occurs when an import is being done.  This should
    allow one to use ldif2db to do an offline replica initialization when
    chain-on-update is configured.
    (cherry picked from commit 39714d3d7916123b1f1ca86ec43342e29a6f5fec)