Use the Python frame safely in _pythonCallstack (pytorch#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: pytorch#88993
Approved by: https://github.com/albanD

torch/csrc/jit/python/python_tracer.cpp

@@ -27,7 +27,7 @@ namespace tracer {
 std::vector<StackEntry> _pythonCallstack() {
   pybind11::gil_scoped_acquire gil;
   PyFrameObject* frame = PyEval_GetFrame();
-  Py_INCREF(frame);
+  Py_XINCREF(frame);
   std::vector<StackEntry> entries;
 
   while (nullptr != frame) {