Panic from (*UEventConn).ReadMsg

[stolowski]

I've been experiencing panic from ReadMsg in automated tests in VMs, the Recvfrom call is missing err check and can return negative value on error. I've proposed #4 that should fix it.

Jun 11 14:38:34 jun111628-661587 snapd[13155]: panic: runtime error: slice bounds out of range
Jun 11 14:38:34 jun111628-661587 snapd[13155]: goroutine 15 [running]:
Jun 11 14:38:34 jun111628-661587 snapd[13155]: panic(0x556b13061ea0, 0xc82000a070)
Jun 11 14:38:34 jun111628-661587 snapd[13155]: /usr/lib/go-1.6/src/runtime/panic.go:481 +0x3ea
Jun 11 14:38:34 jun111628-661587 snapd[13155]: github.com/snapcore/snapd/vendor/github.com/pilebones/go-udev/netlink.(*UEventConn).ReadMsg(0xc820322320, 0x0, 0x0, 0x0, 0x0, 0x0)
Jun 11 14:38:34 jun111628-661587 snapd[13155]: /home/gopath/src/github.com/snapcore/snapd/_build/src/github.com/snapcore/snapd/vendor/github.com/pilebones/go-udev/netlink/conn.go:72 +0x148
Jun 11 14:38:34 jun111628-661587 snapd[13155]: github.com/snapcore/snapd/vendor/github.com/pilebones/go-udev/netlink.(*UEventConn).ReadUEvent(0xc820322320, 0x0, 0x0, 0x0)
Jun 11 14:38:34 jun111628-661587 snapd[13155]: /home/gopath/src/github.com/snapcore/snapd/_build/src/github.com/snapcore/snapd/vendor/github.com/pilebones/go-udev/netlink/conn.go:79 +0x33
Jun 11 14:38:34 jun111628-661587 snapd[13155]: github.com/snapcore/snapd/vendor/github.com/pilebones/go-udev/netlink.(*UEventConn).Monitor.func1(0xc820327f20, 0xc820322320, 0xc820326420, 0x0, 0x0, 0xc8203263c0)
Jun 11 14:38:34 jun111628-661587 snapd[13155]: /home/gopath/src/github.com/snapcore/snapd/_build/src/github.com/snapcore/snapd/vendor/github.com/pilebones/go-udev/netlink/conn.go:109 +0x8a
Jun 11 14:38:34 jun111628-661587 snapd[13155]: created by github.com/snapcore/snapd/vendor/github.com/pilebones/go-udev/netlink.(*UEventConn).Monitor
Jun 11 14:38:34 jun111628-661587 snapd[13155]: /home/gopath/src/github.com/snapcore/snapd/_build/src/github.com/snapcore/snapd/vendor/github.com/pilebones/go-udev/netlink/conn.go:124 +0x239

[pilebones]

Indeed, this error should be catched and checked, that is a oversight.

Thank you a lot for this fix !