Vless fallback: we shoud not create fallback-connection after inbound-connection is closed

[patterniha]

first do the following test to understand better:

1. run this xray config

2. sniff loopback port 8888 with wireshark

3. run this python script
   this script create a connection to xray-vless-inbound and close the connection after 1 seconds

you see after inbound-connection is closed, xray create a connection to fallback and then close it immediately!!!

this is just a waste and should be fixed.

[Fangliding]

等其他的合并吧别发这些有的没的了 谁没事打堆syn flood

[patterniha]

What does it have to do with syn-flood?
in many situations connection is closed before receiving any data.

CC @RPRX

[patterniha]

all other protocols trojan/socks... return error when reading the first data is unsuccessful:

Xray-core/proxy/trojan/server.go

Lines 163 to 166 in 83686eb

	firstLen, err := first.ReadFrom(conn)
	if err != nil {
	return errors.New("failed to read first request").Base(err)
	}

this is forgotten by the author for vless, and should be fixed.