vcl: fix lookup listener assert
vcl: fix lookup listener assert

The following timing may happen:

1. vpp accept new connection, send ACCEPTED to vcl, but vcl has
   not handle it
2. vcl unbind the listener, go into DISCONNECT state
3. vcl handle the ACCEPTED event

Type: fix

Signed-off-by: liuyacan <12ac68815e324e48eabd4f40bba5a7fc8ec28656@corp.netease.com>
Change-Id: If9a96a153af38763db062df133c2fdcc04ceba9b

FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp