Fix bug in Assembler's analytic gradient calculation for built-in constraints when some of the q's are locked.
Fix bug in Assembler's analytic gradient calculation for built-in constraints when some of the q's are locked.

simbody/simbody,scamille/simbody,simbody/simbody,elkingtonmcb/simbody,chrisdembia/simbody,Gjacquenot/simbody,Gjacquenot/simbody,Gjacquenot/simbody,scamille/simbody,robojukie/simbody,elkingtonmcb/simbody,elkingtonmcb/simbody,robojukie/simbody,elen4/simbody,elen4/simbody,scamille/simbody,robojukie/simbody,chrisdembia/simbody,chrisdembia/simbody