mptcp: show all endpoints when no ID is specified

According to 'ip mptcp help', 'endpoint show' can accept no argument:

  ip mptcp endpoint show [ id ID ]

It makes sense to print all endpoints when no filter is used.

So here if the following command is used, all endpoints are printed:

  ip mptcp endpoint show

Same as:

  ip mptcp endpoint

Fixes: 7e0767cd ("add support for mptcp netlink interface")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

ip/ipmptcp.c

@@ -273,7 +273,7 @@ static int mptcp_addr_show(int argc, char **argv)
 	struct nlmsghdr *answer;
 	int ret;
 
-	if (!argv)
+	if (argc <= 0)
 		return mptcp_addr_dump();
 
 	ret = mptcp_parse_opt(argc, argv, &req.n, false);