Bugfix MiriAllocBytes to guarantee different addresses

[Strophox]

Fix in alloc_bytes.rs following #3526

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

[RalfJung]

but this means returned ptrs may overlap

"overlap" isn't quite right, since arguably ranges with with size 0 can never overlap.

But two different allocations can end up at the same address, and that's a problem.

[RalfJung]

but this surprisingly failed all tests:

Why did they fail? Setting the size to 1 should work and avoids duplicating logic so please figure out why it did not work.

[RalfJung]

@bors r+

[bors]

📌 Commit 120efdc has been approved by RalfJung

It is now in the queue for this repository.

[bors]

⌛ Testing commit 120efdc with merge d7ddcaf...

[bors]

☀️ Test successful - checks-actions
Approved by: RalfJung
Pushing d7ddcaf to master...