Fix +1 error in storing krbPrincipalKey LDAP attr.

For principal entries having keys with multiple kvnos (due to use of
-keepold), LDAP backend plugin makes an attempt to store all the keys
having the same kvno into single krbPrincipalKey attribute value.
There is an +1 error in the loop, causing currkvno variable being set
to the just processed value, instead of the next kvno.
As a result, second and all following groups of keys by kvnos are each
stored in two krbPrincipalKey attribute values.

Fixed to use correct kvno value.