ipw2x00: Fix potential NULL dereference in libipw_xmit()

crypt and crypt->ops could be null, so we need to checking null
before dereference
Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/1648797055-25730-1-git-send-email-baihaowen@meizu.com

drivers/net/wireless/intel/ipw2x00/libipw_tx.c

@@ -383,7 +383,7 @@ netdev_tx_t libipw_xmit(struct sk_buff *skb, struct net_device *dev)
 
 		/* Each fragment may need to have room for encryption
 		 * pre/postfix */
-		if (host_encrypt)
+		if (host_encrypt && crypt && crypt->ops)
 			bytes_per_frag -= crypt->ops->extra_mpdu_prefix_len +
 			    crypt->ops->extra_mpdu_postfix_len;