Fixed allocation in WorkItem::getConstant() for 3-element vectors.
Fixed allocation in WorkItem::getConstant() for 3-element vectors.

mpflanzer/Oclgrind,surround-io/oclgrind,surround-io/oclgrind,mpflanzer/Oclgrind,surround-io/oclgrind,bitfusionio/Oclgrind,mpflanzer/Oclgrind,mpflanzer/Oclgrind,bitfusionio/Oclgrind,bitfusionio/Oclgrind,bitfusionio/Oclgrind,surround-io/oclgrind