Check if the iterator is valid before dereferencing it (#72405)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/72405
Fixes #71674.
This shouldn't segfault now:
```
import torch
d = torch.complex64
torch.set_default_dtype(d)
```
Test Plan: Imported from OSS
Reviewed By: jbschlosser
Differential Revision: D34423660
Pulled By: anjali411
fbshipit-source-id: cac92a6f56846f2c0727a120b5f568aa75baa21e
(cherry picked from commit eaab813a0fddced24303b3bd50e4fcdba1516e46)
