mrt: fix bug of general address family rib serialization
mrt: fix bug of general address family rib serialization

allocate a slice with 2 byte length (not 2 byte capacity) for
PutUint16()

fixes #1313

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

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