selftests/net/ipsec: Fix Null pointer dereference in rtattr_pack()

Address Null pointer dereference / undefined behavior in rtattr_pack
(note that size is 0 in the bad case).

Flagged by cppcheck as:
    tools/testing/selftests/net/ipsec.c:230:25: warning: Possible null pointer
    dereference: payload [nullPointer]
    memcpy(RTA_DATA(attr), payload, size);
                           ^
    tools/testing/selftests/net/ipsec.c:1618:54: note: Calling function 'rtattr_pack',
    4th argument 'NULL' value is 0
    if (rtattr_pack(&req.nh, sizeof(req), XFRMA_IF_ID, NULL, 0)) {
                                                       ^
    tools/testing/selftests/net/ipsec.c:230:25: note: Null pointer dereference
    memcpy(RTA_DATA(attr), payload, size);
                           ^
Signed-off-by: Liu Ye <liuye@kylinos.cn>

Link: https://patch.msgid.link/20250116013037.29470-1-liuye@kylinos.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/ipsec.c

@@ -227,7 +227,8 @@ static int rtattr_pack(struct nlmsghdr *nh, size_t req_sz,
 
 	attr->rta_len = RTA_LENGTH(size);
 	attr->rta_type = rta_type;
-	memcpy(RTA_DATA(attr), payload, size);
+	if (payload)
+		memcpy(RTA_DATA(attr), payload, size);
 
 	return 0;
 }