[gnome-font-viewer] Fix a crash when no sample string can be constructed

[Cosimo Cecchi <cosimoc src gnome org>]

commit 1af4733223882c8870040c6333117ade116cbbac
Author: Cosimo Cecchi <cosimoc gnome org>
Date:   Sat Nov 21 14:21:53 2015 -0800

    Fix a crash when no sample string can be constructed
    
    When no sample string can be constructed, we end up trying to measure a
    NULL string, leading to a crash.
    While the user experience when no string can be constructed using the
    glyphs available in the specified font can be improved, at least avoid a
    crash here.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=757680

 src/sushi-font-widget.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/sushi-font-widget.c b/src/sushi-font-widget.c
index 1b7febd..3709db5 100644
--- a/src/sushi-font-widget.c
+++ b/src/sushi-font-widget.c
@@ -636,7 +636,8 @@ sushi_font_widget_draw (GtkWidget *drawing_area,
 
   for (i = 0; i < n_sizes; i++) {
     cairo_set_font_size (cr, sizes[i]);
-    draw_string (self, cr, padding, self->priv->sample_string, &pos_y);
+    if (self->priv->sample_string !=  NULL)
+      draw_string (self, cr, padding, self->priv->sample_string, &pos_y);
     if (pos_y > allocated_height)
       break;
   }