[3.9] bpo-41604: Don't decrement the reference count of the previous user_ptr when set_panel_usertpr fails (GH-21933). (GH-24403)

(cherry picked from commit 3243e8a)

Co-authored-by: Anonymous Maarten <[email protected]>
(cherry picked from commit 3c8d693)

Co-authored-by: Serhiy Storchaka <[email protected]>

Author: miss-islington

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

@@ -433,7 +433,9 @@ _curses_panel_panel_set_userptr(PyCursesPanelObject *self, PyObject *obj)
         /* In case of an ncurses error, decref the new object again */
         Py_DECREF(obj);
     }
-    Py_XDECREF(oldobj);
+    else {
+        Py_XDECREF(oldobj);
+    }
     return PyCursesCheckERR(rc, "set_panel_userptr");
 }