minor optimization - don't do double lookups on vertex mask layer for vert_mask_get(), vert_mask_set().

add an assert because if the mesh is in an invalid state the mask layer can exist but the mask pointer still be NULL (noticed this while looking into a different bug).


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54135 954f8c5b-7b00-dc11-b283-0030488c597c