fix for problem with creating weight-paint preview.

In the case where the modifier stack didnt need deform-verts to calculate,
they would not be available for the preview either.

This fixes a bug caused by r57206 which set mirror to preview so the mirrored
weights would be displayed, but it only worked when there was an armature after it, see [#35545].


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57419 954f8c5b-7b00-dc11-b283-0030488c597c