Reject oversized sockaddr payloads received over privsep IPC.

This is not an issue on its own but may permit lateral movement or memory corruption
inside the privsep environment after another bug.

diff by Stuart Thomas <stuart.thomas@triageforge.co.uk>

usr.sbin/smtpd/mproc.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: mproc.c,v 1.48 2025/10/30 16:46:19 gilles Exp $	*/
+/*	$OpenBSD: mproc.c,v 1.49 2026/05/26 22:48:13 gilles Exp $	*/
 
 /*
  * Copyright (c) 2012 Eric Faurot <eric@faurot.net>
@@ -589,6 +589,8 @@ m_get_sockaddr(struct msg *m, struct sockaddr *sa)
 	size_t len;
 
 	m_get_size(m, &len);
+	if (len > sizeof(struct sockaddr_storage))
+		m_error("sockaddr size too large");
 	m_get(m, sa, len);
 }