fix eval stack register X error in print_uw

examples/screencodes.p8

@@ -33,5 +33,18 @@ main {
         c64scr.print_ub(c1)
         c64scr.print("\nscreencode z=")
         c64scr.print_ub(c2)
+        c64scr.print("\n")
+
+        check_eval_stack()
+    }
+
+
+
+    sub check_eval_stack() {
+        if X!=255 {
+            c64scr.print("stack x=")
+            c64scr.print_ub(X)
+            c64scr.print(" error!\n")
+        }
     }
 }