bugfix: memory leak

lualib/dns.lua

@@ -250,7 +250,9 @@ function dns.server(server, port)
 		assert(server, "Can't get nameserver")
 	end
 	dns_server = socket.udp(function(data, sz, from)
-		resolve(skynet.tostring(data,sz))
+		local str = skynet.tostring(data,sz)
+		skynet.trash(data,sz)
+		resolve(str)
 	end)
 	socket.udp_connect(dns_server, server, port or 53)
 	return server

lualib/skynet.lua

@@ -398,6 +398,7 @@ skynet.pack = assert(c.pack)
 skynet.packstring = assert(c.packstring)
 skynet.unpack = assert(c.unpack)
 skynet.tostring = assert(c.tostring)
+skynet.trash = assert(c.trash)
 
 local function yield_call(service, session)
 	watching_session[session] = service