Fix out-of-bounds read in c_substitute():

http://seclists.org/oss-sec/2016/q4/612

Patch copied from upstream source repository:

https://github.com/tcsh-org/tcsh/commit/6a542dc4fb2ba26518a47e9b3a9bcd6a91b94596

From 6a542dc4fb2ba26518a47e9b3a9bcd6a91b94596 Mon Sep 17 00:00:00 2001
From: christos <christos>
Date: Fri, 2 Dec 2016 16:59:28 +0000
Subject: [PATCH] Fix out of bounds read (Brooks Davis) (reproduce by starting
 tcsh and hitting tab at the prompt)

---
 ed.chared.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/ed.chared.c b/ed.chared.c
index 1277e53..310393e 100644
--- ed.chared.c
+++ ed.chared.c
@@ -750,7 +750,7 @@ c_substitute(void)
     /*
      * If we found a history character, go expand it.
      */
-    if (HIST != '\0' && *p == HIST)
+    if (p >= InputBuf && HIST != '\0' && *p == HIST)
 	nr_exp = c_excl(p);
     else
         nr_exp = 0;
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