interrogate: fix uninitialized var, fixes sporadic nb_true_divide

This was a regression introduced by fa6c066b2f21856036cb24d482a7152406c6c2e2 (which in turn fixed #529)

dtool/src/interrogate/interfaceMakerPythonNative.h

@@ -121,7 +121,7 @@ private:
   public:
     std::string _answer_location;
     WrapperType _wrapper_type;
-    int _min_version;
+    int _min_version = 0;
     std::string _wrapper_name;
     std::set<FunctionRemap*> _remaps;
     bool _keep_method;