Auto merge of #53804 - RalfJung:ptr-invalid, r=nagisa

fix some uses of pointer intrinsics with invalid pointers

[Found by miri](https://github.com/solson/miri/pull/446):

* `Vec::into_iter` calls `ptr::read` (and the underlying `copy_nonoverlapping`) with an unaligned pointer to a ZST. [According to LLVM devs](https://bugs.llvm.org/show_bug.cgi?id=38583), this is UB because it contradicts the metadata we are attaching to that pointer.
* `HashMap` creation calls `ptr:.write_bytes` on a NULL pointer with a count of 0. This is likely not currently UB *currently*, but it violates the rules we are setting in https://github.com/rust-lang/rust/pull/53783, and we might want to exploit those rules later (e.g. with more `nonnull` attributes for LLVM).

    Probably what `HashMap` really should do is use `NonNull::dangling()` instead of 0 for the empty case, but that would require a more careful analysis of the code.

It seems like ideally, we should do a review of usage of such intrinsics all over libstd to ensure that they use valid pointers even when the size is 0. Is it worth opening an issue for that?