pythongh-95808: Add missing early returns in _asynciomodule.c (python#95809)

Author: 1st1

Modules/_asynciomodule.c

@@ -631,15 +631,16 @@ create_cancelled_error(FutureObj *fut)
     } else {
         exc = PyObject_CallOneArg(asyncio_CancelledError, msg);
     }
-    PyException_SetContext(exc, fut->fut_cancelled_exc);
-    Py_CLEAR(fut->fut_cancelled_exc);
     return exc;
 }
 
 static void
 future_set_cancelled_error(FutureObj *fut)
 {
     PyObject *exc = create_cancelled_error(fut);
+    if (exc == NULL) {
+        return;
+    }
     PyErr_SetObject(asyncio_CancelledError, exc);
     Py_DECREF(exc);
 }