layout: Be quiet about trivialities

Commit 8b82993dde added a noisy warning
to gtk_distribute_natural_allocation to quiet a
compiler warning. It turn out that the file chooser
managed to trigger this warning, so make it a quiet
return.

gtk/gtksizerequest.c

@@ -698,7 +698,9 @@ gtk_distribute_natural_allocation (int               extra_space,
   int    i;
 
   g_return_val_if_fail (extra_space >= 0, 0);
-  g_return_val_if_fail (n_requested_sizes > 0, 0);
+
+  if (n_requested_sizes == 0)
+    return extra_space;
 
   spreading = g_newa (guint, n_requested_sizes);