MOTION/MPC Walkgen: Fix aligned pointers issues for Vector4d

Change-Id: I7ff425139249bc30062f987e19a59082dd0c8717
Reviewed-on: http://gerrit.aldebaran.lan:8080/19014
Tested-by: agarcia <agarcia@aldebaran-robotics.com>
Reviewed-by: agarcia <agarcia@aldebaran-robotics.com>
Tested-by: gerrit
Reviewed-by: mdegourcuff <mdegourcuff@aldebaran-robotics.com>
Tested-by: mdegourcuff <mdegourcuff@aldebaran-robotics.com>