fix crash when socket_listener receiving invalid data

[vhqtvn]

Socket_listener cause Telegraf crash when receiving invalid data if NewStreamContentDecoder returning error. decoder is nil at that time that cause scnr.Scan to dereference null pointer.

E! [inputs.socket_listener] Read error: %vunexpected EOF
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x5951da]

goroutine 25 [running]:
bufio.(*Scanner).Scan(0xc000669ec0, 0xbfed4a168ea3030f)
	/usr/local/go/src/bufio/scan.go:214 +0x7a
[global_tags]
github.com/influxdata/telegraf/plugins/inputs/socket_listener.(*streamSocketListener).read(0xc0005c4800, 0x33d9d80, 0xc000010008)
	/go/src/github.com/influxdata/telegraf/plugins/inputs/socket_listener/socket_listener.go:124 +0x1ae
github.com/influxdata/telegraf/plugins/inputs/socket_listener.(*streamSocketListener).listen.func1(0xc0001107f0, 0xc0005c4800, 0x33d9d80, 0xc000010008)
	/go/src/github.com/influxdata/telegraf/plugins/inputs/socket_listener/socket_listener.go:74 +0x6b
created by github.com/influxdata/telegraf/plugins/inputs/socket_listener.(*streamSocketListener).listen
	/go/src/github.com/influxdata/telegraf/plugins/inputs/socket_listener/socket_listener.go:72 +0x26b

The fix is easy: just stop when we have an error.

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.