Fixed bug that crashes zombie siege in context where z cannot spawn

mods/ENVIRONMENT/mcl_zombie_sieges/init.lua

@@ -10,10 +10,14 @@ local function spawn_zombies(self)
 		local p = vector.offset(nn[i%#nn],0,1,0)
 		if check_spawn_pos(p) then
 			local m = mcl_mobs.spawn(p,"mobs_mc:zombie")
+			if m then
 				local l = m:get_luaentity()
 				mcl_mobs:gopath(m:get_luaentity(), self.pos)
 				table.insert(self.mobs, m)
 				self.health_max = self.health_max + l.health
+			else
+				--minetest.log("Failed to spawn zombie at location: " .. minetest.pos_to_string(p))
+			end
 		end
 	end
 end