Check for closed clientConn earlier in the handleProxyConnection
function in order to prevent a crash:

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x28 pc=0x40394d]

goroutine 1550625 [running]:
main.handleProxyConnection(0xc20cbca950, 0xc20cc5b0a0, 0xc, 0x1bb)
    /root/go-any-proxy/any_proxy.go:427 +0x7d
main.handleConnection(0xc20cbca950)
    /root/go-any-proxy/any_proxy.go:496 +0xef
created by main.main
    /root/go-any-proxy/any_proxy.go:283 +0x308