`Multiheader::preallocate` causes an uninitialized value to be dereferenced

[djkoloski]

In src/sys/socket/mod.rs, Multiheader::preallocate allocates a vec or MaybeUninit::uninit() and then passes each address.as_mut_ptr() to pack_mhdr_to_receive. pack_mhdr_to_receive in turn performs (*address).as_mut_ptr(), which results in undefined behavior.

To support this desired behavior SockaddrLikePriv must change the signature of as_mut_ptr to fn(*mut Self) -> *mut libc::sockaddr and use ptr::addr_of_mut! to access fields in implementations. It does not appear that performing *mut S as *mut libc::sockaddr is possible because () implements SockaddrLike but returns null_mut() from its SockaddrLikePriv implementation.