emacs-27 109eb1e: Fix undefined behavior when fetching glyphs from the display vector.

[Philipp Stephani]

branch: emacs-27
commit 109eb1e7e29455418b40ca00bf5dad3e61e5fc78
Author: Philipp Stephani <phst@google.com>
Commit: Philipp Stephani <phst@google.com>

    Fix undefined behavior when fetching glyphs from the display vector.
    
    You can trigger this rather obscure bug by enabling selective display
    if the second glyph in its display vector has an invalid face.  For
    example, evaluate
    
    (set-display-table-slot standard-display-table
                            'selective-display [?A (?B . invalid)])
    
    and then enable selective display.
    
    * src/xdisp.c (next_element_from_display_vector): Check whether next
    glyph code is valid before accessing it.
---
 src/xdisp.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/src/xdisp.c b/src/xdisp.c
index ac706d0..71a5f1c 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -8221,10 +8221,10 @@ next_element_from_display_vector (struct it *it)
            next_face_id = it->dpvec_face_id;
          else
            {
-             int lface_id =
-               GLYPH_CODE_FACE (it->dpvec[it->current.dpvec_index + 1]);
+              Lisp_Object gc = it->dpvec[it->current.dpvec_index + 1];
+              int lface_id = GLYPH_CODE_P (gc) ? GLYPH_CODE_FACE (gc) : 0;
 
-             if (lface_id > 0)
+              if (lface_id > 0)
                next_face_id = merge_faces (it->w, Qt, lface_id,
                                            it->saved_face_id);
            }