sizerequest: Avoid a corner case

gcc 11 warns that the code isn't safe when
n_requested_sizes is 0. Add a precondition check
to make it clear that that never happens.

gtk/gtksizerequest.c

@@ -698,6 +698,7 @@ 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);
 
   spreading = g_newa (guint, n_requested_sizes);