Don't close connection if Listen* fails

[vincentbernat]

When ListenUDP/ListenPacket fail, the connection is nil and therefore, we cannot call Close() on it.

Before:

 16:47 CEST ❱ ./goflow-v3.4.2-8-g0c2503d67cc8-linux-x86_64 -kafka=false -nf.port 9995 -nfl=false -sflow=false
INFO[0000] Starting GoFlow
INFO[0000] Listening on UDP :9995                        Type=NetFlow
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x7f644a]

goroutine 23 [running]:
github.com/cloudflare/goflow/v3/utils.UDPRoutine(0x90460d, 0x7, 0xc00021c3c0, 0x1, 0x0, 0x0, 0x270b, 0x0, 0x99b358, 0xc00021a930, ...)
        /home/bernat/code/free/goflow/utils/utils.go:291 +0x140a
github.com/cloudflare/goflow/v3/utils.(*StateNetFlow).FlowRoutine(0xc000224900, 0x1, 0x0, 0x0, 0x270b, 0x0, 0x2, 0x0)
        /home/bernat/code/free/goflow/utils/netflow.go:358 +0x15c
main.main.func2(0xc000224900, 0xc00021e260)
        /home/bernat/code/free/goflow/cmd/goflow/goflow.go:134 +0x226
created by main.main
        /home/bernat/code/free/goflow/cmd/goflow/goflow.go:129 +0x3da

After:

 16:48 CEST ❱ ./goflow-v3.4.2-8-g0c2503d67cc8-linux-x86_64 -kafka=false -nf.port 9995 -nfl=false -sflow=false
INFO[0000] Starting GoFlow
INFO[0000] Listening on UDP :9995                        Type=NetFlow
FATA[0000] Fatal error: could not listen to UDP (listen udp :9995: bind: address already in use)