Potential Undefined Behaviors in `Arc<T>`/`Rc<T>` impls of `from_value` on OOM

The `SharedPointer::alloc` implementation for `sync::Arc<T>` and `rc::Rc<T>` in `rkyv/src/impls/alloc/rc/atomic.rs` (and `rc.rs`) does not check if the allocator returns a null pointer on OOM (Out of Memory).

This null pointer can flow through to `SharedPointer::from_value`, which calls `Box::from_raw(ptr)` with the null pointer. This triggers undefined behavior when utilizing safe deserialization APIs (such as `rkyv::from_bytes` or `rkyv::deserialize_using`) if an OOM condition occurs during the allocation of the shared pointer.

The issue is reachable through safe code and violates Rust's safety guarantees.