Fix CVE-2017-1000369:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-1000369
https://www.qualys.com/2017/06/19/stack-clash/stack-clash.txt

Patch adapted from upstream source repository:

https://git.exim.org/exim.git/commit/65e061b76867a9ea7aeeb535341b790b90ae6c21

From 65e061b76867a9ea7aeeb535341b790b90ae6c21 Mon Sep 17 00:00:00 2001
From: "Heiko Schlittermann (HS12-RIPE)" <hs@schlittermann.de>
Date: Wed, 31 May 2017 23:08:56 +0200
Subject: [PATCH] Cleanup (prevent repeated use of -p/-oMr to avoid mem leak)

---
 doc/doc-docbook/spec.xfpt |  3 ++-
 src/src/exim.c            | 19 +++++++++++++++++--
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/src/src/exim.c b/src/src/exim.c
index 67583e58..88e11977 100644
--- a/src/exim.c
+++ b/src/exim.c
@@ -3106,7 +3106,14 @@ for (i = 1; i < argc; i++)
 
       /* -oMr: Received protocol */
 
-      else if (Ustrcmp(argrest, "Mr") == 0) received_protocol = argv[++i];
+      else if (Ustrcmp(argrest, "Mr") == 0)
+
+        if (received_protocol)
+          {
+          fprintf(stderr, "received_protocol is set already\n");
+          exit(EXIT_FAILURE);
+          }
+        else received_protocol = argv[++i];
 
       /* -oMs: Set sender host name */
 
@@ -3202,7 +3209,15 @@ for (i = 1; i < argc; i++)
 
     if (*argrest != 0)
       {
-      uschar *hn = Ustrchr(argrest, ':');
+      uschar *hn;
+
+      if (received_protocol)
+        {
+        fprintf(stderr, "received_protocol is set already\n");
+        exit(EXIT_FAILURE);
+        }
+
+      hn = Ustrchr(argrest, ':');
       if (hn == NULL)
         {
         received_protocol = argrest;
-- 
2.13.1