mrt: bug fix of decoding BGP4MPHeader
mrt: bug fix of decoding BGP4MPHeader

Signed-off-by: ISHIDA Wataru <15703620d0b11e835ab4f34ebf88f2f40cce783e@lab.ntt.co.jp>

a16/gobgp,osrg/gobgp,Dipsingh/gobgp,osrg/gobgp,YujiOshima/gobgp,a16/gobgp,YujiOshima/gobgp,ramrunner/gobgp,ramrunner/gobgp,YujiOshima/gobgp,h-naoto/gobgp,tamihiro/gobgp,a16/gobgp,fujita/gobgp,ishidawataru/gobgp,j-kato/gobgp,fujita/gobgp,fujita/gobgp,tamihiro/gobgp,Dipsingh/gobgp,tamihiro/gobgp,tamihiro/gobgp,Dipsingh/gobgp,ishidawataru/gobgp,a16/gobgp,a16/gobgp,a16/gobgp,osrg/gobgp,postfix/gobgp,postfix/gobgp,h-naoto/gobgp,osrg/gobgp,ishidawataru/gobgp,h-naoto/gobgp,postfix/gobgp,ramrunner/gobgp,a16/gobgp,fujita/gobgp,j-kato/gobgp,j-kato/gobgp