af_packet: remove unnecessary BUG_ON() in tpacket_destruct_skb

If skb is NULL, then stack trace is thrown anyway on dereference.
Therefore, the stack trace triggered by BUG_ON is duplicate.

Signed-off-by: Daniel Borkmann <danborkmann@googlemail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>