Cherry-pick #8347 to 6.4: Reduce errors on filebeat syslog stop

[jsoriano]

Cherry-pick of PR #8347 to 6.4 branch. Original message:

Fix a couple of errors seen when syslog input is stopped.

In case the input couldn't be started (e.g. port was already in use), there was a nil pointer reference when trying to stop it:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x12facac]

goroutine 81 [running]:
github.com/elastic/beats/filebeat/inputsource/udp.(*Server).Stop(0xc42008a100)
	/go/src/github.com/elastic/beats/filebeat/inputsource/udp/server.go:118 +0x7c
github.com/elastic/beats/filebeat/input/syslog.(*Input).Stop(0xc42008a140)
	/go/src/github.com/elastic/beats/filebeat/input/syslog/input.go:189 +0xeb
github.com/elastic/beats/filebeat/input.(*Runner).stop(0xc42025e2a0)
	/go/src/github.com/elastic/beats/filebeat/input/input.go:173 +0xcd
github.com/elastic/beats/filebeat/input.(*Runner).Start.func1.1(0xc420277410, 0xc42025e2a0)
	/go/src/github.com/elastic/beats/filebeat/input/input.go:128 +0x39
github.com/elastic/beats/filebeat/input.(*Runner).Start.func1(0xc420277410, 0xc42025e2a0)
	/go/src/github.com/elastic/beats/filebeat/input/input.go:133 +0x5c
created by github.com/elastic/beats/filebeat/input.(*Runner).Start
	/go/src/github.com/elastic/beats/filebeat/input/input.go:125 +0x16e

In any case, on stop, this error was logged lots of times:

2018-09-18T17:37:42.818Z	ERROR	[udp]	udp/server.go:99	Error reading from the socket *net.OpError read udp 127.0.0.1:9000: use of closed network connection	{"address": "localhost:9000"}