Error getting variable in PUAE core running CD32.

[JSensebe]

Using the puae_libretro core, when attempting to build a session which loads a CD32 game, the following error is issued:

Exception ignored on calling ctypes callback function: <bound method DictEnvironmentDriver.environment of <libretro.drivers.environment.composite.CompositeEnvironmentDriver object at 0x0000025A936C6CF0>>
Traceback (most recent call last):
  File "C:\Users\jsens\AppData\Roaming\Python\Python312\site-packages\libretro\drivers\environment\dict.py", line 44, in environment
    return self._envcalls[envcall](data)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\jsens\AppData\Roaming\Python\Python312\site-packages\libretro\drivers\environment\default.py", line 97, in <lambda>
    EnvironmentCall.GET_VARIABLE: lambda data: self._get_variable(
                                               ^^^^^^^^^^^^^^^^^^^
  File "C:\Users\jsens\AppData\Roaming\Python\Python312\site-packages\libretro\drivers\environment\composite.py", line 527, in _get_variable
    result = self._options.get_variable(string_at(var.key) if var.key else None)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\jsens\AppData\Roaming\Python\Python312\site-packages\libretro\drivers\options\dict.py", line 75, in get_variable
    value = string_at(self._options_us[key].default_value)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Program Files\Python312\Lib\ctypes\__init__.py", line 525, in string_at
    return _string_at(ptr, size)
           ^^^^^^^^^^^^^^^^^^^^^
OSError: exception: access violation reading 0x0000000000000000

The system appears to run fine after the error.

The variable (key) is b'puae_cart_file'. It is in self._options_us but not in self._variables.
self._options_us[key].default_value is set to None, which means it's essentially trying to get string_at(None). There probably should be a check for this case, but I don't know off-hand what a reasonable return value would be (b'', None, or even b'None'?).

[JSensebe]

The 3DO core opera_libretro has the same issue when getting b'opera_bios' and b'opera_font'. It could be preventing the core from finding a default BIOS file. As such, this core might be better for testing a potential fix.