Fix Open Message optional parameter length == 1 GoBGP crashes issues.
Fix Open Message optional parameter length == 1 GoBGP crashes issues.

tamihiro/gobgp,Dipsingh/gobgp,osrg/gobgp,j-kato/gobgp,a16/gobgp,osrg/gobgp,a16/gobgp,ishidawataru/gobgp,j-kato/gobgp,fujita/gobgp,ishidawataru/gobgp,osrg/gobgp,tamihiro/gobgp,fujita/gobgp,tamihiro/gobgp,fujita/gobgp,Dipsingh/gobgp,a16/gobgp,a16/gobgp,Dipsingh/gobgp,osrg/gobgp,fujita/gobgp,j-kato/gobgp,ishidawataru/gobgp,a16/gobgp,a16/gobgp,tamihiro/gobgp,a16/gobgp