fix array index out of bounds in mip fraction

This fixes a small bug with an out-of-bounds array index when determining the MIP fraction for FastSim hadronic showers. This caused the code to grab values out of neighboring memory. The bug only affects hadrons with energy > 3 TeV, so the effect on simulation results should be small.

Example of buggy behavior (from a few 5 TeV pion events):
Begin processing the 1st record. Run 1, Event 1, LumiSection 1 at 14-Jul-2014 21:06:57.152 CEST
CalorimetryManager onHcal 6.92421e-310
CalorimetryManager onHcal 6.92421e-310
Begin processing the 2nd record. Run 1, Event 2, LumiSection 1 at 14-Jul-2014 21:07:02.608 CEST
CalorimetryManager onHcal 0.99857
CalorimetryManager onHcal 0.99618

Example of fixed behavior:
Begin processing the 1st record. Run 1, Event 1, LumiSection 1 at 14-Jul-2014 21:31:38.048 CEST
CalorimetryManager onHcal 0.09604
CalorimetryManager onHcal 0.09294
Begin processing the 2nd record. Run 1, Event 2, LumiSection 1 at 14-Jul-2014 21:31:42.721 CEST
CalorimetryManager onHcal 0.01353
CalorimetryManager onHcal 0.03486