Even CLS fix

It's a fix of a minor problem. The problem was due to unexpected interaction between two
algorithms: "Left-right ambiguity algorithm" (used to digitize the strips in case of even cluster size) and
"Shifting algorithm" (used to simulate the asymmetric cluster size, i.e. to shift the clusters).
Detailed description of the problem, could be find in RPC DPG report:
https://indico.cern.ch/event/337922/session/0/contribution/10/material/slides/0.pdf