bpo-40048: Fix _PyCode_InitOpcache() error path (pythonGH-19691)

If _PyCode_InitOpcache() fails in _PyEval_EvalFrameDefault(), use
"goto exit_eval_frame;" rather than "return NULL;" to exit the
function in a consistent state. For example, tstate->frame is now
reset properly.

Python/ceval.c

@@ -1297,7 +1297,7 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
         co->co_opcache_flag++;
         if (co->co_opcache_flag == OPCACHE_MIN_RUNS) {
             if (_PyCode_InitOpcache(co) < 0) {
-                return NULL;
+                goto exit_eval_frame;
             }
 #if OPCACHE_STATS
             opcache_code_objects_extra_mem +=