fix signed/unsigned mismatch warning
fix signed/unsigned mismatch warning

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