Modifiers: pass the ob->data to the modifier if the mesh param is NULL

This enables the modifiers to access things like vertex groups. Care should
be taken to not modifier the mesh itself in this case.

source/blender/modifiers/intern/MOD_lattice.c

@@ -109,11 +109,11 @@ static void deformVerts(ModifierData *md, const ModifierEvalContext *ctx,
                         int numVerts)
 {
 	LatticeModifierData *lmd = (LatticeModifierData *) md;
-
+	struct Mesh *mesh_src = mesh ? mesh : ctx->object->data;
 
 	modifier_vgroup_cache(md, vertexCos); /* if next modifier needs original vertices */
 
-	lattice_deform_verts(lmd->object, ctx->object, mesh,
+	lattice_deform_verts(lmd->object, ctx->object, mesh_src,
 	                     vertexCos, numVerts, lmd->name, lmd->strength);
 }