listbase: Clear any newly set adjustments

The scrolling code assumes the adjustment values are up to date or
it crashes and before we've run size_allocate() we haven't update them.

Fixes a crash in the gtk-demo scrollinfo that would set the adjustments
with random values (via ScrolledWindow.set_child()) and then scroll in a
tick callback right before the (first) size_allocate().

gtk/gtklistbase.c

@@ -731,6 +731,8 @@ gtk_list_base_set_adjustment (GtkListBase    *self,
 
   if (adjustment == NULL)
     adjustment = gtk_adjustment_new (0.0, 0.0, 0.0, 0.0, 0.0, 0.0);
+  else
+    gtk_adjustment_configure (adjustment, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0);
   g_object_ref_sink (adjustment);
 
   gtk_list_base_clear_adjustment (self, orientation);