Check for deletion before moving vert in dyntopo collapse edge

Fixes [#33964] Dyntopo crash with edge collapse + undo
projects.blender.org/tracker/?func=detail&aid=33964&group_id=9&atid=498

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54109 954f8c5b-7b00-dc11-b283-0030488c597c