config: allow unconfigured peer ASN
config: allow unconfigured peer ASN

The commit ab953211 prohibits the unconfigured peer ASN but it breaks
the unnumbered BGP feature. The old behavior needs to be reverted.

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

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