Fixed a bug in the GroundMotionValuesGetter

A misprint in the getter caused all of the risk numbers to be wrong, see https://bugs.launchpad.net/oq-engine/+bug/1329654. Since the differences are only in the last digits, this went unnoticed until now. The tests are running here: https://ci2.openquake.org/job/zdevel_oq-engine/401