bpo-45262, asyncio: Fix cache of the running loop holder (GH-28796)

Prevent use-after-free of running loop holder via cache.

Author: HiassofT

Misc/NEWS.d/next/Library/2021-10-07-14-04-10.bpo-45262.HqF71Z.rst

@@ -0,0 +1 @@
+Prevent use-after-free in asyncio. Make sure the cached running loop holder gets cleared on dealloc to prevent use-after-free in get_running_loop

Modules/_asynciomodule.c

@@ -3239,6 +3239,9 @@ new_running_loop_holder(PyObject *loop)
 static void
 PyRunningLoopHolder_tp_dealloc(PyRunningLoopHolder *rl)
 {
+    if (cached_running_holder == (PyObject *)rl) {
+        cached_running_holder = NULL;
+    }
     Py_CLEAR(rl->rl_loop);
     PyObject_Free(rl);
 }