selftests: mptcp: connect: fix maybe-uninitialize warn

This warning can be seen with GCC 15.2:

  mptcp_connect.c: In function ‘main_loop’:
  mptcp_connect.c:1422:37: warning: ‘peer’ may be used uninitialized [-Wmaybe-uninitialized]
   1422 |                 if (connect(fd, peer->ai_addr, peer->ai_addrlen))
        |                                 ~~~~^~~~~~~~~
  mptcp_connect.c:1377:26: note: ‘peer’ was declared here
   1377 |         struct addrinfo *peer;
        |                          ^~~~

This variable is set in sock_connect_mptcp() in some conditions. If not,
this helper returns an error, and the program stops. So this is a false
positive, but better removing it by initialising peer to NULL.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260205-net-mptcp-misc-fixes-6-19-rc8-v2-4-c2720ce75c34@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: matttbe

tools/testing/selftests/net/mptcp/mptcp_connect.c

@@ -1296,8 +1296,8 @@ void xdisconnect(int fd)
 
 int main_loop(void)
 {
+	struct addrinfo *peer = NULL;
 	int fd = 0, ret, fd_in = 0;
-	struct addrinfo *peer;
 	struct wstate winfo;
 
 	if (cfg_input && cfg_sockopt_types.mptfo) {