When I repeatedly add ImuFactor isam will core dump

[薛海涛]

I replaced LevenbergMarquardtOptimizer as isam2 in ImuFactorExample.cpp, and I commented code about GPS, then the process will core dump and report ill pose.

That means when I add three PriorFactor on pose, velocity and bias then I repeatedly add CombinedImuFactor then the process will core dump.

Maybe I made some mistakes, thanks for your helping.