ScriptFu: fix #12696 improper reffing of object proxies
This fixes one bug and also makes a debugging message less scary to users.
The logic for detecting errors in proxy reffing is heuristic, and not always correct. It gives a debug message when a proxy refcount is not 1,3,5,... but 2 is also valid. The debug messages require diagnosis by a developer.
Alternatively, we might change the code so a persistent plugin instance does not keep temporary procedure instances. Which is what makes many object proxy instances have a ref count of 2.