bug#41232: [PATCH] Avoid a segfault when processing disable-eval specs

["Clément Pit-Claudel" <cpitclaudel@gmail.com>]

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The following code causes Emacs to segfault:

  (insert (propertize "A" 'display '(disable-eval . nil)))

This is due to this code, which takes an XCAR without checking whether the spec is a cons:

  /* Support (disable-eval PROP) which is used by enriched.el.  */
  if (CONSP (spec) && EQ (XCAR (spec), Qdisable_eval))
    {
      enable_eval = false;
      spec = XCAR (XCDR (spec));
    }

The attached patch fixes this.

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From b0954112bbe2dc124041a6439a4cc371fd9809fd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Cl=C3=A9ment=20Pit-Claudel?= <clement.pitclaudel@live.com>
Date: Wed, 13 May 2020 10:37:05 -0400
Subject: [PATCH] Fix a crash in handle_display_spec

* src/xdisp.c (handle_display_spec): Check that the cdr of the
disable-eval spec is a cons before taking its car.
---
 src/xdisp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/xdisp.c b/src/xdisp.c
index d65bb38869..cf15f579b5 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -5120,7 +5120,7 @@ handle_display_spec (struct it *it, Lisp_Object spec, Lisp_Object object,
   if (CONSP (spec) && EQ (XCAR (spec), Qdisable_eval))
     {
       enable_eval = false;
-      spec = XCAR (XCDR (spec));
+      spec = CONSP (XCDR (spec)) ? XCAR (XCDR (spec)) : Qnil;
     }
 
   if (CONSP (spec)
-- 
2.17.1