Fix len check

[stolowski]

Followup to my yesterday's PR - fix for length check - casting to int in the invalid len test gives negative value and it only manifests itself with a panic on 32-bit systems:

=== RUN   TestParseUdevEvent
--- FAIL: TestParseUdevEvent (0.00s)
panic: runtime error: slice bounds out of range [recovered]
    panic: runtime error: slice bounds out of range

goroutine 8 [running]:
panic(0x81692a0, 0x18420010)
    /usr/lib/go-1.6/src/runtime/panic.go:481 +0x326
testing.tRunner.func1(0x184c8e40)
    /usr/lib/go-1.6/src/testing/testing.go:467 +0x14f
panic(0x81692a0, 0x18420010)
    /usr/lib/go-1.6/src/runtime/panic.go:443 +0x3fd
github.com/snapcore/snapd/osutil/udev/netlink.parseUdevEvent(0x18414370, 0x42, 0x50, 0x0, 0x0, 0x0)
    github.com/snapcore/snapd/osutil/udev/netlink/_test/_obj_test/uevent.go:133 +0x846
github.com/snapcore/snapd/osutil/udev/netlink.ParseUEvent(0x18414370, 0x42, 0x50, 0x0, 0x0, 0x0)
    github.com/snapcore/snapd/osutil/udev/netlink/_test/_obj_test/uevent.go:178 +0x153
github.com/snapcore/snapd/osutil/udev/netlink.TestParseUdevEvent(0x184c8e40)
    /tmp/static-unit-tests/src/github.com/snapcore/snapd/osutil/udev/netlink/uevent_test.go:178 +0x866
testing.tRunner(0x184c8e40, 0x8236c04)
    /usr/lib/go-1.6/src/testing/testing.go:473 +0x8f
created by testing.RunTests
    /usr/lib/go-1.6/src/testing/testing.go:582 +0x6f2
exit status 2
FAIL    github.com/snapcore/snapd/osutil/udev/netlink   0.005s

[pilebones]

Ok for me thanks !