Minor bug fix to autodiff.h
Minor bug fix to autodiff.h

Change-Id: Ib41050a2f2ba1898c71ff19d74f8eca2496212c0

ysonggit/ceres-solver,entylop/ceres-solver,entylop/ceres-solver,DougFirErickson/ceres-solver,whuaegeanse/ceres-solver,ikoruk/ceres-solver,entylop/ceres-solver,DougFirErickson/ceres-solver,whuaegeanse/ceres-solver,kartikkumar/ceres-solver,kartikkumar/ceres-solver,ysonggit/ceres-solver,ikoruk/ceres-solver,ysonggit/ceres-solver,whuaegeanse/ceres-solver,whuaegeanse/ceres-solver,DougFirErickson/ceres-solver,timfel/ceres-bin,kartikkumar/ceres-solver,stonier/ceres_solver,entylop/ceres-solver,ysonggit/ceres-solver,stonier/ceres_solver,ikoruk/ceres-solver,timfel/ceres-bin,ikoruk/ceres-solver,timfel/ceres-bin,stonier/ceres_solver,DougFirErickson/ceres-solver,ikoruk/ceres-solver,timfel/ceres-bin,stonier/ceres_solver,kartikkumar/ceres-solver