Use the Python frame safely in _pythonCallstack (#88993)
Currently, the result of `PyEval_GetFrame()` is piped straight to `Py_INCREF`. However, `PyEval_GetFrame` [may return null](https://docs.python.org/3/c-api/reflection.html#c.PyEval_GetFrame), which seems to be the case sometimes, when calling `_pythonCallstack` from another thread. This is handled in the subsequent `while (nullptr != frame)` block, but `Py_INCREF`, called before it, [doesn't handle this case](https://docs.python.org/3/c-api/refcounting.html#c.Py_INCREF), so the program segfaults. The safe form of `Py_INCREF` is `Py_XINCREF`, so use that instead ([docs](https://docs.python.org/3/c-api/refcounting.html#c.Py_XINCREF)).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88993
Approved by: https://github.com/albanD
