Packet130Update is a CraftBukkit construct.  TileEntitySign is all nms glory itself.  As I see it, I'd modularize the line length checks in TileEntitySign to be a publicly accessible static call which can be chained from a CraftSign method that also does a null check.  Then consolidate all sign line sanitizing (Packet130UpdateSign, TileEntitySign, any any others if possible) through CraftSign accordingly.

This will keep all sign line sanitizing in a central location, CraftSign, that is then dependent directly upon the native TileEntitySign implementation which will be easily caught upon updates as appropriate.