Fix the kinsol problem in inverseKinBackend.m

Fix #517. The `inverseKinBackend.m` has been broken for the `MultipleKinematicConstraint`. The previous test did not catch this as we either use mex version of inverseKinBackend, or abort when snopt is not found. But if the user installs student snopt and call inverseKinTraj, such error will occur.