[cppyy] Fix memory leak in TemplateProxy::Instantiate()

The `PyObject * exact` is set to be the return value of
`PyObject_GetItem()`, which returns a new reference that needs to be
dereferenced when not used anymore. Otherwise, memory will leak.

This could be a problem in code that uses lots of template
instantiations.

Author: guitargeek

bindings/pyroot/cppyy/CPyCppyy/src/TemplateProxy.cxx

@@ -296,6 +296,7 @@ PyObject* TemplateProxy::Instantiate(const std::string& fname,
         }
 
     // cleanup
+        Py_XDECREF(exact);
         Py_DECREF(pyresname);
         Py_DECREF(pycachename);