net/tap: avoid memcpy with null argument

Calling memcpy with a null pointer even if zero length is
undefined, so check if data_length is zero.
Problem reported by Gcc analyzer.

Fixes: 7c25284 ("net/tap: add netlink back-end for flow API")
Cc: stable@dpdk.org

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Ferruh Yigit <ferruh.yigit@amd.com>

drivers/net/tap/tap_netlink.c

@@ -302,7 +302,8 @@ tap_nlattr_add(struct nlmsghdr *nh, unsigned short type,
 	rta = (struct rtattr *)NLMSG_TAIL(nh);
 	rta->rta_len = RTA_LENGTH(data_len);
 	rta->rta_type = type;
-	memcpy(RTA_DATA(rta), data, data_len);
+	if (data_len > 0)
+		memcpy(RTA_DATA(rta), data, data_len);
 	nh->nlmsg_len = NLMSG_ALIGN(nh->nlmsg_len) + RTA_ALIGN(rta->rta_len);
 }