nfc: netlink: Add check on NFC_ATTR_VENDOR_DATA

NFC_ATTR_VENDOR_DATA is an optional vendor_cmd argument.
The current code was potentially using a non existing argument
leading to potential catastrophic results.

Cc: stable@vger.kernel.org
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

net/nfc/netlink.c

@@ -1518,8 +1518,8 @@ static int nfc_genl_vendor_cmd(struct sk_buff *skb,
 	if (!dev || !dev->vendor_cmds || !dev->n_vendor_cmds)
 		return -ENODEV;
 
+	if (info->attrs[NFC_ATTR_VENDOR_DATA]) {
 		data = nla_data(info->attrs[NFC_ATTR_VENDOR_DATA]);
-	if (data) {
 		data_len = nla_len(info->attrs[NFC_ATTR_VENDOR_DATA]);
 		if (data_len == 0)
 			return -EINVAL;