Fix crash when sheep replaces grass

mods/ENTITIES/mobs_mc/sheep.lua

@@ -86,6 +86,9 @@ mobs:register_mob("mobs_mc:sheep", {
 	-- Properly regrow wool after eating grass
 	on_replace = function(self, pos, oldnode, newnode)
 		self.gotten = false
+		if not self.color or not colors[self.color] then
+			self.color = "unicolor_white"
+		end
 		self.drops = {
 		{name = mobs_mc.items.mutton_raw,
 		chance = 1,