Update frame.h

include/pocketpy/frame.h

@@ -47,8 +47,9 @@ struct ValueStack {
     PyVar& peek(int n){ return _sp[-n]; }
     PyVar peek(int n) const { return _sp[-n]; }
     void push(PyVar v){ *_sp++ = v; }
+    void push(std::nullptr_t) { memset(_sp++, 0, sizeof(PyVar)); }
     void pop(){ --_sp; }
-    PyVar popx(){ return *--_sp; }
+    PyVar popx(){ --_sp; return *_sp; }
     ArgsView view(int n){ return ArgsView(_sp-n, _sp); }
     void shrink(int n){ _sp -= n; }
     int size() const { return _sp - _begin; }