bpo-41604: Don't decrement the reference count of the previous user_ptr when set_panel_usertpr fails (GH-21933)

Author: madebr

Misc/NEWS.d/next/Library/2020-08-21-15-24-14.bpo-41604.rTXleO.rst

@@ -0,0 +1,2 @@
+Don't decrement the reference count of the previous user_ptr when
+set_panel_userptr fails.

Modules/_curses_panel.c

@@ -456,7 +456,9 @@ _curses_panel_panel_set_userptr_impl(PyCursesPanelObject *self,
         /* In case of an ncurses error, decref the new object again */
         Py_DECREF(obj);
     }
-    Py_XDECREF(oldobj);
+    else {
+        Py_XDECREF(oldobj);
+    }
 
     _curses_panel_state *state = PyType_GetModuleState(cls);
     return PyCursesCheckERR(state, rc, "set_panel_userptr");