crash in SGWilson player_xxx_portal_loop

Within the 3.5second timeout, from player_porkland_portal_pre to player_porkland_portal_loop, the chair is not protected against destruction, hence when it is destroyed by a giant, the target in

            local target = GetClosestInstWithTag("porkland_portal", inst, 5)

will be nil and cause a crash.

Adding a if check can avoid this.