`pdb` commands may try to access non-existing frames

Bug report

Bug description:

➜  cpython git:(main) ./python.exe
Python 3.15.0a0 (heads/main:919c7e806e, Sep  8 2025, 17:24:03) [Clang 17.0.0 (clang-1700.0.13.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> breakpoint()
> <python-input-0>(1)<module>()
(Pdb) breakpoint()
(Pdb) ll
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    breakpoint()
    ~~~~~~~~~~^^
  File "/Users/hyi/prs/cpython/Lib/bdb.py", line 94, in wrapper
    ret = func(frame, *args)
  File "/Users/hyi/prs/cpython/Lib/bdb.py", line 154, in opcode_callback
    frame.f_trace(frame, 'opcode', None)
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/hyi/prs/cpython/Lib/bdb.py", line 289, in trace_dispatch
    return self.dispatch_opcode(frame, arg)
           ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
  File "/Users/hyi/prs/cpython/Lib/bdb.py", line 406, in dispatch_opcode
    self.user_opcode(frame)
    ~~~~~~~~~~~~~~~~^^^^^^^
  File "/Users/hyi/prs/cpython/Lib/pdb.py", line 547, in user_line
    self.interaction(frame, None)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/Users/hyi/prs/cpython/Lib/pdb.py", line 736, in interaction
    self._cmdloop()
    ~~~~~~~~~~~~~^^
  File "/Users/hyi/prs/cpython/Lib/pdb.py", line 601, in _cmdloop
    self.cmdloop()
    ~~~~~~~~~~~~^^
  File "/Users/hyi/prs/cpython/Lib/cmd.py", line 149, in cmdloop
    stop = self.onecmd(line)
  File "/Users/hyi/prs/cpython/Lib/pdb.py", line 1038, in onecmd
    return cmd.Cmd.onecmd(self, line)
           ~~~~~~~~~~~~~~^^^^^^^^^^^^
  File "/Users/hyi/prs/cpython/Lib/cmd.py", line 227, in onecmd
    return func(arg)
  File "/Users/hyi/prs/cpython/Lib/pdb.py", line 2156, in do_longlist
    filename = self.curframe.f_code.co_filename
               ^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'f_code'
>>> 

CPython versions tested on:

3.15, CPython main branch

Operating systems tested on:

Other

Linked PRs

• gh-138642

從 Lib/pdb.py 的 do_longlist 開始，檢查透過命令迴圈到達的 frame 狀態；traceback 也指出 Lib/bdb.py 是呼叫路徑的一部分。重現 breakpoint() 和 ll 序列，然後確認在不存在目前 frame 時，pdb 命令不再引發 AttributeError。