gh-143007: fix crash .TextIOWrapper.seek with a custom __int__ method detaches

[yihong0618]

this patch fix crash .TextIOWrapper.seek with a custom int method detaches

add textiowrapper_parse_cookie instead of check detect again to make the code clear

• Issue: Null pointer dereference in TextIOWrapper.seek via re-entrant __int__ #143007

[cmaloney]

I think this isn't the right place to add the check, the _PyFile_Flush just before this could result in the same issue, and the cookie's buffer / decoding into it isn't what actually breaks (the buffer in this function is locally allocated, the cookie should be valid the whole time of this function).

I think this should do a CHECK_ATTACHED like check just before the res = PyObject_CallMethodOneArg(self->buffer, &_Py_ID(seek), posobj); as that's what is doing the self->buffer usage/call/dereference.

[yihong0618]

Yes but it is my first solution
an rethink that we can forget add in the future in other place

CHECK_ATTACHED and check it in the middle of the code is not match its name