table: fix 2byte AS peer helper functions

If AS_PATH attribute doesn't exist, just return.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
server: handle peer not capable of four byte AS number

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>