[3.12] gh-108487: Change assert that should've been DEOPT_IF (#108509)

* Remove assert that should've been DEOPT_IF

The assert(method != NULL) in CALL_NO_KW_LIST_APPEND is wrong --
this condition should lead to a deoptimization, and indeed there
is a DEOPT_IF two lines later that will trigger if method == NULL.

This would crash in a devious repro scenario (first seen live
in boto3 tests) when compiled with assertions enabled.
In a production version there is no crash, so impact is limited.

(The crash also appears in main; I will prepare a separate PR.)

* Add back a different assert(self != NULL)

* 📜🤖 Added by blurb_it.

---------

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>

Author: gvanrossum

Misc/NEWS.d/next/Core and Builtins/2023-08-26-04-33-18.gh-issue-108487.aUFxqf.rst

@@ -0,0 +1 @@
+Change an assert that would cause a spurious crash in a devious case that should only trigger deoptimization.

Python/bytecodes.c

@@ -2992,9 +2992,9 @@ dummy_func(
         inst(CALL_NO_KW_LIST_APPEND, (unused/1, unused/2, method, self, args[oparg] -- unused)) {
             assert(kwnames == NULL);
             assert(oparg == 1);
-            assert(method != NULL);
             PyInterpreterState *interp = _PyInterpreterState_GET();
             DEOPT_IF(method != interp->callable_cache.list_append, CALL);
+            assert(self != NULL);
             DEOPT_IF(!PyList_Check(self), CALL);
             STAT_INC(CALL, hit);
             if (_PyList_AppendTakeRef((PyListObject *)self, args[0]) < 0) {