Fix an issue on the number of clocks introduced in commit 5c1cb14e.

src/uppaalta.ml

@@ -639,7 +639,7 @@ struct
   (* this should be used from another function that caches the results *)
   (* TODO some clocks already have their constant known, cache them *)
   let _lubounds ta state =
-    let n = nb_clocks ta in
+    let n = VarContext.size ta.clocks in
     let lb, ub = Array.make n 0, Array.make n 0 in
     for cl=0 to n-1 do
       let lexpr, uexpr = Array.fold_left