master 509dec9: Fix out-of-bounds access in xdisp.c

[Eli Zaretskii]

branch: master
commit 509dec902c45bc11cd8ff6664795b471d451742c
Author: Eli Zaretskii <eliz@gnu.org>
Commit: Eli Zaretskii <eliz@gnu.org>

    Fix out-of-bounds access in xdisp.c
    
    * src/xdisp.c (produce_stretch_glyph): Avoid indexing it->stack
    with a negative index.  Reported by Po Lu <luangruo@yahoo.com>.
---
 src/xdisp.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/xdisp.c b/src/xdisp.c
index b2eeb11..45b5025 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -30391,7 +30391,8 @@ produce_stretch_glyph (struct it *it)
   if (width > 0 && height > 0 && it->glyph_row)
     {
       Lisp_Object o_object = it->object;
-      Lisp_Object object = it->stack[it->sp - 1].string;
+      Lisp_Object object =
+       it->sp > 0 ? it->stack[it->sp - 1].string : it->string;
       int n = width;
 
       if (!STRINGP (object))