C++11 fix in LinearContainerFactor
C++11 fix in LinearContainerFactor

Implicit conversion from shared_ptr to bool is disabled, so have
to explicitly compare to the nullptr constant.

malcolmreynolds/GTSAM,malcolmreynolds/GTSAM,malcolmreynolds/GTSAM,malcolmreynolds/GTSAM,malcolmreynolds/GTSAM,malcolmreynolds/GTSAM