pgraph: mark TransformState as invalid in destructor

This, in combination with ebef9f3de8, makes it easier to catch issues in code that is accidentally letting get temporary TransformState objects get destroyed.  It does not affect release builds.

panda/src/pgraph/transformState.cxx

@@ -96,6 +96,10 @@ TransformState::
   // true now, probably we've been double-deleted.
   nassertv(get_ref_count() == 0);
   _cache_stats.add_num_states(-1);
+
+#ifndef NDEBUG
+  _flags = F_is_invalid | F_is_destructing;
+#endif
 }
 
 /**