bgpd: Fix aggregated routes are withdrawn abnormally.

bgpd: Fix aggregated routes are withdrawn abnormally.

    The withdraw message and announcement message of a prefix are received continuously within 50ms, which may lead to abnormal aggregation route reference count.
    Steps to reproduce:
    --------------------------
    step1:
    local config aggregate route 111.0.0.0/24
    received route：111.0.0.1/32 111.0.0.02/32
    ref_count:2

    step2:
    peer withdraw 111.0.0.1/32 and network 111.0.0.1/32 in 50ms
    received route：111.0.0.1/32 111.0.0.02/32
    ref_count:1

    step3:
    peer withdraw 111.0.0.1/32
    received route：111.0.0.02/32
    ref_count:0
    aggregate route will be withdrawn abnormally

Signed-off-by: liuze03 <liuze03@baidu.com>