peer: prefix-limit calls peer.adjRibIn.Count only if enabled
peer: prefix-limit calls peer.adjRibIn.Count only if enabled

No need to call peer.adjRibIn.Count() if prefix-limit isn't enabled
(not configured).

Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

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