msg/async: Don't dec(msgr_active_connections) if conn still in accepting_conns.

msg/async: Don't dec(msgr_active_connections) if conn still in accepting_conns.

I found this bug during test:
"AsyncMessenger::Worker-0": {
        "msgr_active_connections": 18446744073709313353,
    },

This because before AsyncMessenger::accept_conn, the conn met fault and
stop this conn. In unregister_conn, it will
dec(msgr_active_connections). But the conn it don't inc(msgr_active_connections).
Only erase from accepting_conns, it will inc(msgr_active_connections).

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>