NFC: llcp: integer underflow in nfc_llcp_set_remote_gb()

If gb_len is less than 3 it would cause an integer underflow and
possibly memory corruption in nfc_llcp_parse_gb_tlv().

I removed the old test for gb_len == 0.  I also removed the test for
->remote_gb == NULL.  It's not possible for ->remote_gb to be NULL and
we have already dereferenced ->remote_gb_len so it's too late to test.

The old test return -ENODEV but my test returns -EINVAL.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/nfc/llcp/llcp.c

@@ -549,14 +549,13 @@ int nfc_llcp_set_remote_gb(struct nfc_dev *dev, u8 *gb, u8 gb_len)
 		pr_err("No LLCP device\n");
 		return -ENODEV;
 	}
+	if (gb_len < 3)
+		return -EINVAL;
 
 	memset(local->remote_gb, 0, NFC_MAX_GT_LEN);
 	memcpy(local->remote_gb, gb, gb_len);
 	local->remote_gb_len = gb_len;
 
-	if (local->remote_gb == NULL || local->remote_gb_len == 0)
-		return -ENODEV;
-
 	if (memcmp(local->remote_gb, llcp_magic, 3)) {
 		pr_err("MAC does not support LLCP\n");
 		return -EINVAL;