Fix segfault with EM, DD and group scheme

Fix segfault with EM, DD and group scheme

Resetting to an old DD state during EM would leave the cg sorting array
used with the group scheme in an invalid state. This could cause
out of bounds vector access one DD step after rejecting an EM step.

When merging this into master branch, prefer to change to use ssize
rather than the static_cast.

Fixes #2813

Change-Id: I7f13b46d7ff5352ce41838b813c46f2e90c93b1c