Blender Git Loki

MOD_curve: do not compute mesh when not needed.

This modifier only uses mesh to get vgroup, which is only needed in case
modified object is indeed a mesh! Building a mesh from curve here is not
only useless and time-consuming, it will also easily fail the assert
about same number of vertices!

Also, use MOD_get_vgroup() helper in modifier code itself and pass
reluting MDeformVert & index to BKE_curve's curve_deform_verts(),
this is simpler and avoids duplicating vgroup handling code.

Also fixes crash when used on lattice.

Related to T57972.