Another one fix for control buf handling in udp_xmit

To be able to put data to control buffer, it should be allocated and
assigned to message header. Do that unconditionally before message
build. Clear the control buffer fields just before message sending if
control buffer empty.

This change should finally fix 4830a20
(use address from the last received UDP packet in UDP messages response)
and bug introduced in 90368df (Fix
control buffer handling in udp_xmit)

network.c

@@ -286,6 +286,8 @@ void udp_xmit (struct buffer *buf, struct tunnel *t)
      * OKAY, now send a packet with the right SAref values.
      */
     memset(&msgh, 0, sizeof(struct msghdr));
+    msgh.msg_control = cbuf;
+    msgh.msg_controllen = sizeof(cbuf);
 
     if (gconfig.ipsecsaref && t->refhim != IPSEC_SAREF_NULL) {
 	cmsg = CMSG_FIRSTHDR(&msgh);
@@ -303,9 +305,6 @@ void udp_xmit (struct buffer *buf, struct tunnel *t)
     }
 
 #ifdef LINUX
-    msgh.msg_control = cbuf;
-    msgh.msg_controllen = sizeof(cbuf);
-
     if (t->my_addr.ipi_addr.s_addr){
 	struct in_pktinfo *pktinfo;
 
@@ -325,20 +324,17 @@ void udp_xmit (struct buffer *buf, struct tunnel *t)
 
 	finallen += cmsg->cmsg_len;
     }
-
-    msgh.msg_controllen = finallen;
 #endif
 
     /*
      * Some OS don't like assigned buffer with zero length (e.g. OpenBSD),
      * some OS don't like empty buffer with non-zero length (e.g. Linux).
      * So make them all happy by assigning control buffer only if we really
-     * have something there.
+     * have something there and zero both fields otherwise.
      */
-    if (finallen) {
-        msgh.msg_control = cbuf;
-        msgh.msg_controllen = finallen;
-    }
+    msgh.msg_controllen = finallen;
+    if (!finallen)
+        msgh.msg_control = NULL;
 
     iov.iov_base = buf->start;
     iov.iov_len  = buf->len;