Fix mob egg double-spawns

If you spawn a mob clicking on a wall, two mobs will be spawned.

To reproduce: face a stack of stones, with a spawn egg click on the side of a stone. It does not happen when you click the top of a node, because spawning below fails and only the second one succeeds.

mods/ENTITIES/mcl_mobs/init.lua

@@ -615,7 +615,7 @@ function mcl_mobs.register_egg(mob_id, desc, background_color, overlay_color, ad
 
 				pos.y = pos.y - 1
 				local mob = mcl_mobs.spawn(pos, mob_name)
-				if not object then
+				if not mob then
 					pos.y = pos.y + 1
 					mob = mcl_mobs.spawn(pos, mob_name)
 					if not mob then return end