Fix potential crash during sprite creation

src/caches.lua

@@ -39,6 +39,8 @@ local function cache_groups(group, groups)
 
 	for _, item in ipairs(items) do
 		local def = reg_items[item]
+
+		if def then
 			local tiles = def.tiles or def.tile_images
 			local texture = true_str(def.inventory_image) and def.inventory_image --or tiles[1]
 
@@ -54,12 +56,15 @@ local function cache_groups(group, groups)
 				if c == lim then break end
 			end
 		end
+	end
 
+	if c > 1 then
 		sprite = sprite:gsub("WxH", px .. "x" .. px * c)
 
 		i3.groups[group].sprite = sprite
 		i3.groups[group].count = c
 	end
+end
 
 local function get_item_usages(item, recipe, added)
 	if is_group(item) then