Looking at how the path is accessed, the cast is safe.

The lifetime of the delta object is the same as the lifetime of the `git_diff_list` object itself and the delta doesn't keep a back-reference nor is it refcounted, so if the `diff` object goes away, the delta is not going to be valid any longer. (By the way, the `patch` object does keep a back reference to the `diff` with refcounting, so a patch can be held safely and if the patch is held, then the delta structures will remain valid.)