ltsmin: register dead only if it is an atomic proposition

* spot/ltsmin/ltsmin.cc: Here.  This fixes 5a441e1b.

spot/ltsmin/ltsmin.cc

@@ -1118,7 +1118,8 @@ namespace spot
     // twa::ap() works.
     for (auto ap: *to_observe)
       res->register_ap(ap);
-    res->register_ap(dead);
+    if (dead.is(op::ap))
+      res->register_ap(dead);
     return res;
   }