Initialize pointers in RIGID and SHOCK

src/RIGID/compute_erotate_rigid.cpp

src/RIGID/compute_ke_rigid.cpp

src/RIGID/compute_rigid_local.cpp

src/RIGID/fix_ehex.cpp

src/RIGID/fix_ehex.h

src/RIGID/fix_rigid.cpp

src/RIGID/fix_rigid_nh.cpp

src/RIGID/fix_rigid_nh_small.cpp

src/RIGID/fix_rigid_small.cpp

src/RIGID/fix_shake.cpp

src/SHOCK/fix_append_atoms.cpp

src/SHOCK/fix_msst.cpp

src/SHOCK/fix_nphug.cpp

src/SHOCK/fix_wall_piston.cpp