Fix decoding of AS-PATH for long-ish attr

[netixx]

The current implementation incurs counter wrap when the path is longer than an uint8.

I debugged using the following test-case, but I am not sure how to include it properly in the test-suite:

func TestASPathDecode(t *testing.T) {
	update := []byte{2, 72, 0, 0, 18, 29, 0, 0, 218, 13, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43, 0, 2, 82, 43}

	asPathAttr  := &ASPathAttr{}
	err := asPathAttr.Decode(PathAttrFlags(1<<6), update)
	if err != nil {
		t.Errorf("failed to decode: %s", err)
	}
}

[jwhited]

thanks!