[3.10] bpo-45355: More use of sizeof(_Py_CODEUNIT) (GH-28720). (GH-28721)

(cherry picked from commit 252b7bcb236dc261f3af1275bc90f9a303d9648f)
diff --git a/Objects/frameobject.c b/Objects/frameobject.c
index 8974d37..d02cf9d 100644
--- a/Objects/frameobject.c
+++ b/Objects/frameobject.c
@@ -275,8 +275,8 @@ marklines(PyCodeObject *code, int len)
     }
 
     while (PyLineTable_NextAddressRange(&bounds)) {
-        assert(bounds.ar_start/2 < len);
-        linestarts[bounds.ar_start/2] = bounds.ar_line;
+        assert(bounds.ar_start/(int)sizeof(_Py_CODEUNIT) < len);
+        linestarts[bounds.ar_start/sizeof(_Py_CODEUNIT)] = bounds.ar_line;
     }
     return linestarts;
 }