Disable multires in sculpt mode if dynamic-topology mode is enabled

sculpt_multires_active() now returns NULL if dynamic topology is
enabled. Fixes bug #33718:
projects.blender.org/tracker/?func=detail&aid=33718&group_id=9&atid=498

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53610 954f8c5b-7b00-dc11-b283-0030488c597c