Auto merge of rust-lang#3625 - Strophox:miri-allocation-fix, r=RalfJung

Bugfix `MiriAllocBytes` to guarantee different addresses

Fix in `alloc_bytes.rs` following rust-lang/miri#3526

Currently when an allocation of `size == 0` is requested we return a `std::ptr::without_provenance_mut(align)`, but this means returned `ptr`s may overlap, which breaks things.

Author: bors