Bugfix #5008

With the addition of vertexgroup support for Lattices, the option
'create vertex groups from closest bone' crashed due to reading NULL.

Fix provided & tested by Joshua.

source/blender/src/editdeform.c

@@ -302,6 +302,9 @@ void remove_vert_def_nr (Object *ob, int def_nr, int vertnum)
 			dvert = lt->dvert + vertnum;
 	}
 	
+	if(dvert==NULL)
+		return;
+	
 	/* for all of the deform weights in the
 	 * deform vert
 	 */