fix: alloc(0) call is UB

[EmilyMatt]

When calling alloc(), the layout must be non-zero, otherwise we fall into undefined behaviour.
This is documented in the function's docs:
https://doc.rust-lang.org/beta/src/core/alloc/global.rs.html#152

We can just make a dangling pointer for those cases, and not deallocate it

Found by running miri on tests::test_send_sync