Fixed essential dynamics (ED) continuation from .cpt for reference=average

ED runs where the reference and average structure indices are the same can
crash when continued from checkpoint. For these cases (where reference =
average atom indices) obviously the set of atomic positions for the reference
and average structures is always identical. Therefore, only one of the two
structures is stored (which is the average structure edi->sav). When reading
the old values of these structures from the checkpoint file, edi->sav.x_old
therefore needs to be copied both to xstart and xfit in init_edsam().

Change-Id: Ieb1f029f4a927999dfb4579ee7c3bebe15071dc8