Bluetooth: Mesh: Fix proxy advertising set sending Mesh messages

When start to sending proxy advertising, will also process in
send_pending_adv, but the bt_mesh_adv_get_by_tag will directly return
buffer from bt_mesh_adv_queue or bt_mesh_relay_queue, which case
mesh messages sent on different sets, can cause peer replay attack.

Signed-off-by: Lingao Meng <menglingao@xiaomi.com>