Auto merge of #815 - RalfJung:memory-audit, r=RalfJung

don't call Memory::get without checking the pointer first

Also avoid Memory::get if we just need to know align/size.

I audited all uses of `alloc_id`; the rest should be fine (and we can kill a bunch of them once https://github.com/rust-lang/rust/pull/62257 lands).

diff --cc src/shims/foreign_items.rs
Simple merge