Fix undo crash related to hyperlinks. [#730466]

2014-05-22  Andreas J. Guelzow <aguelzow@pyrshep.ca>

	* src/commands.c (cmd_hyperlink_undo): use posiitons rather than cells
	(cmd_hyperlink_redo): ditto
	(cmd_hyperlink_finalize): ditto
	(cb_cmd_hyperlink_find_cells): ditto