python: Clear AttributeError sometimes created in callback_defined (R…

…HBZ#1693283).

When we test if a callback is defined in the Python code, we try to
convert the callback name into a Python attribute using
PyObject_GetAttrString.  If this fails (indicating the callback isn't
defined) it sets the global PyErr indicator to some random error
message.

At a later point we may test the global PyErr and report this error
unexpectedly, which is what happened in
https://bugzilla.redhat.com/show_bug.cgi?id=1693283#c7

A simple way to fix this is to call PyErr_Clear() along the not found
path.

Thanks: Tomáš Golembiovský and Junqin Zhou.

plugins/python/python.c

@@ -92,8 +92,10 @@ callback_defined (const char *name, PyObject **obj_rtn)
   assert (module != NULL);
 
   obj = PyObject_GetAttrString (module, name);
-  if (!obj)
+  if (!obj) {
+    PyErr_Clear (); /* Clear the AttributeError from testing attr. */
     return 0;
+  }
   if (!PyCallable_Check (obj)) {
     nbdkit_debug ("object %s isn't callable", name);
     Py_DECREF (obj);