[gtk/fix-fontrendering-demo] gtk-demo: Avoid a segfault

[Matthias Clasen <matthiasc src gnome org>]

commit 54daad387853a125c9614545718f1abce26050bc
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Sep 9 12:33:08 2022 -0400

    gtk-demo: Avoid a segfault
    
    The code in the fontrendering demo is a bit sloppy
    and assumes that we always get a single run when
    appending a sequence of 4 chars and 4 spaces.
    
    That is not in general true, such as for Emoji.
    
    Instead of working harder to handle Emoji here,
    just give up and fall back to 'a'.
    
    Fixes: #5166

 demos/gtk-demo/fontrendering.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)
---
diff --git a/demos/gtk-demo/fontrendering.c b/demos/gtk-demo/fontrendering.c
index 283523f481..9f85d2c03b 100644
--- a/demos/gtk-demo/fontrendering.c
+++ b/demos/gtk-demo/fontrendering.c
@@ -217,30 +217,39 @@ update_image (void)
         text = " ";
 
       ch = g_utf8_get_char (text);
-
       str = g_string_new ("");
+      layout = pango_layout_new (context);
 
+retry:
       for (i = 0; i < 4; i++)
         {
           g_string_append_unichar (str, ch);
           g_string_append_unichar (str, 0x200c);
         }
 
-      layout = pango_layout_new (context);
       pango_layout_set_font_description (layout, desc);
       pango_layout_set_text (layout, str->str, -1);
-      g_string_free (str, TRUE);
       pango_layout_get_extents (layout, &ink, &logical);
       pango_extents_to_pixels (&logical, NULL);
 
+      iter = pango_layout_get_iter (layout);
+      run = pango_layout_iter_get_run (iter);
+
+      if (run->glyphs->num_glyphs < 8)
+        {
+          /* not a good char to use */
+          g_string_truncate (str, 0);
+          ch = 'a';
+          goto retry;
+        }
+
+      g_string_free (str, TRUE);
+
       surface = cairo_image_surface_create (CAIRO_FORMAT_ARGB32, logical.width * 3 / 2, 4*logical.height);
       cr = cairo_create (surface);
       cairo_set_source_rgb (cr, 1, 1, 1);
       cairo_paint (cr);
 
-      iter = pango_layout_get_iter (layout);
-      run = pango_layout_iter_get_run (iter);
-
       cairo_set_source_rgb (cr, 0, 0, 0);
       for (i = 0; i < 4; i++)
         {