Fix _PyGen_yf()

Issue #28782: Fix a bug in the implementation ``yield from`` when checking
if the next instruction is YIELD_FROM. Regression introduced by WORDCODE
(issue #26647).

Reviewed by Serhiy Storchaka and Yury Selivanov.

Python/ceval.c

@@ -2043,6 +2043,7 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
             f->f_stacktop = stack_pointer;
             why = WHY_YIELD;
             /* and repeat... */
+            assert(f->f_lasti >= (int)sizeof(_Py_CODEUNIT));
             f->f_lasti -= sizeof(_Py_CODEUNIT);
             goto fast_yield;
         }