pythongh-139988: fix a leak when failing to create a Union type (python#139990)

Author: picnixz

Misc/NEWS.d/next/Core_and_Builtins/2025-10-12-11-00-06.gh-issue-139988.4wi51t.rst

@@ -0,0 +1,2 @@
+Fix a memory leak when failing to create a :class:`~typing.Union` type.
+Patch by Bénédikt Tran.

Objects/unionobject.c

@@ -474,11 +474,13 @@ _Py_union_from_tuple(PyObject *args)
     }
     if (PyTuple_CheckExact(args)) {
         if (!unionbuilder_add_tuple(&ub, args)) {
+            unionbuilder_finalize(&ub);
             return NULL;
         }
     }
     else {
         if (!unionbuilder_add_single(&ub, args)) {
+            unionbuilder_finalize(&ub);
             return NULL;
         }
     }