FIx bug in World::reset() and World::getSkeleton(const char*)
FIx bug in World::reset() and World::getSkeleton(const char*)

dartsim/dart,axeisghost/DART6motionBlur,dartsim/dart,axeisghost/DART6motionBlur,dartsim/dart,dartsim/dart,axeisghost/DART6motionBlur,dartsim/dart