NFC: st21nfca: fix double free

The variable nfcid_skb is not changed in the callee nfc_hci_get_param()
if error occurs. Consequently, the freed variable nfcid_skb will be
freed again, resulting in a double free bug. Set nfcid_skb to NULL after
releasing it to fix the bug.
Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/nfc/st21nfca/core.c

@@ -708,6 +708,7 @@ static int st21nfca_hci_complete_target_discovered(struct nfc_hci_dev *hdev,
 							NFC_PROTO_FELICA_MASK;
 		} else {
 			kfree_skb(nfcid_skb);
+			nfcid_skb = NULL;
 			/* P2P in type A */
 			r = nfc_hci_get_param(hdev, ST21NFCA_RF_READER_F_GATE,
 					ST21NFCA_RF_READER_F_NFCID1,