combobox: Add workaround so evolution doesn't crash

Removing the NULL checks in 7f60cab47d
caused evolution to crash. It chains up with NULL and that used to work.

This patch should be reverted for GTK 4.0.

https://bugzilla.gnome.org/show_bug.cgi?id=729496

gtk/gtkcombobox.c

@@ -5505,6 +5505,11 @@ gtk_combo_box_get_preferred_width (GtkWidget *widget,
   gint                   child_min, child_nat;
   GtkBorder              padding;
   gfloat                 arrow_scaling;
+  gint                   dummy;
+
+  /* https://bugzilla.gnome.org/show_bug.cgi?id=729496 */
+  if (natural_size == NULL)
+    natural_size = &dummy;
 
   child = gtk_bin_get_child (GTK_BIN (widget));