Add kccsrv_add_repsFrom() possibility of NULL res argument

Add kccsrv_add_repsFrom() possibility of NULL res argument

We need the ability to utilize this function in a different
manner.  KCC intra-site topology has already vetted the
replica as being appropriate to produce a repsFrom from.
We do not want kccsrv_add_repsFrom() to produce further
checking as was the case for simple topology.   Thus if
we pass a NULL (res) parameter this extra check will
be skipped.

Signed-off-by: Andrew Tridgell <tridge@samba.org>