gh-120811: Fix reference leak upon _PyContext_Exit failure (#120812)

Co-authored-by: Kumar Aditya <[email protected]>

Author: ZeroIntensity

Misc/NEWS.d/next/Library/2024-06-21-14-32-56.gh-issue-120811.eBmVTV.rst

@@ -0,0 +1 @@
+Fix possible memory leak in :meth:`contextvars.Context.run`.

Python/context.c

@@ -661,6 +661,7 @@ context_run(PyContext *self, PyObject *const *args,
         ts, args[0], args + 1, nargs - 1, kwnames);
 
     if (_PyContext_Exit(ts, (PyObject *)self)) {
+        Py_XDECREF(call_result);
         return NULL;
     }