Fix: dangling pointer caused use-after-free

The old code only worked when built-in nodes are only unregistered
at most once while Blender is running. However, this is not the case
when running certain unit tests such as `AbstractHierarchy*`
in `blender_test`.

Found by Sybren, thanks.

source/blender/blenkernel/intern/node.cc

@@ -1380,6 +1380,7 @@ static void node_free_type(void *nodetype_v)
   }
 
   delete nodetype->fixed_declaration;
+  nodetype->fixed_declaration = nullptr;
 
   /* Can be null when the type is not dynamically allocated. */
   if (nodetype->free_self) {