Patch Painting crashing on bad data

Sometimes theere will be a troublesome painting that will crash the serverupon being loaded. this should hopefully keep the server from crashing.

mods/painting/init.lua

@@ -507,6 +507,9 @@ minetest.register_alias("canvas", "painting:canvas_16")
 
 -- fixes the colours which were set by pairs
 local function fix_eldest_grid(data)
+	if data == nil then
+		return
+	end
 	for y in pairs(data) do
 		local xs = data[y]
 		for x in pairs(xs) do