Fix NULL deref in async yield from cleanup

Treat a deleted `StopAsyncIteration.value` as `None` when handling
delegated iterator exhaustion.

Add a regression test covering immediate exhaustion and exhaustion
after yielding an item.

Author: 1st1

Lib/test/test_yield_from_async.py

@@ -1693,6 +1693,32 @@ class TestPEP828Extras(unittest.TestCase):
     parity-checked against ``test_yield_from``.
     """
 
+    @_async_test
+    async def test_custom_iterator_deleted_return_value(self):
+        class Iterator:
+            def __init__(self, remaining):
+                self.remaining = remaining
+
+            def __aiter__(self):
+                return self
+
+            async def __anext__(self):
+                if self.remaining:
+                    self.remaining -= 1
+                    return 42
+                exc = StopAsyncIteration(99)
+                del exc.value
+                raise exc
+
+        async def gen(count):
+            result = yield from Iterator(count)
+            self.assertIsNone(result)
+
+        for count in (0, 1):
+            with self.subTest(count=count):
+                self.assertEqual([item async for item in gen(count)],
+                                 [42] * count)
+
     @_async_test
     async def test_delegate_exception(self):
         yielded_first = sentinel("yielded_first")

Modules/_testinternalcapi/test_cases.c.h

@@ -10,7 +10,6 @@
 #include "pycore_abstract.h"      // _PyIndex_Check()
 #include "pycore_audit.h"         // _PySys_Audit()
 #include "pycore_backoff.h"
-#include "pycore_call.h"          // _Py_METH_CALL_FLAGS
 #include "pycore_cell.h"          // PyCell_GetRef()
 #include "pycore_ceval.h"         // _PyEval_LazyImportName(), _PyEval_LazyImportFrom()
 #include "pycore_code.h"
@@ -1987,7 +1986,12 @@ dummy_func(
 
             int matches = PyErr_GivenExceptionMatches(exc_value, PyExc_StopAsyncIteration);
             if (matches) {
-                value = PyStackRef_FromPyObjectNew(((PyStopAsyncIterationObject *)exc_value)->value);
+                PyObject *ret = ((PyStopAsyncIterationObject *)exc_value)->value;
+                if (ret == NULL) {
+                    value = PyStackRef_None;
+                } else {
+                    value = PyStackRef_FromPyObjectNew(ret);
+                }
                 DECREF_INPUTS();
             }
             else {