bpo-40826: Add _Py_EnsureTstateNotNULL() macro (GH-20571)

Add _Py_EnsureTstateNotNULL(tstate) macro: call Py_FatalError() if
tstate is NULL, the error message contains the current function name.
diff --git a/Python/errors.c b/Python/errors.c
index 70365aa..5d17256 100644
--- a/Python/errors.c
+++ b/Python/errors.c
@@ -1426,7 +1426,7 @@
 _PyErr_WriteUnraisableMsg(const char *err_msg_str, PyObject *obj)
 {
     PyThreadState *tstate = _PyThreadState_GET();
-    assert(tstate != NULL);
+    _Py_EnsureTstateNotNULL(tstate);
 
     PyObject *err_msg = NULL;
     PyObject *exc_type, *exc_value, *exc_tb;