MET Trigger: Add missing fabs() to monitoring function
@khamano noticed in the webdisplay of recent reprocessings (e.g. ATR-20397 and http://atlas.web.cern.ch/Atlas/project/trigger/trigcaf/www/ATR-20397/) that the negative Ex, Ey events are all at 0.
This is because of a missing fabs() call in an underflow check, which is fixed here.