Double-free in `vmem` storage reachable from safe Rust

When the vmem feature is enabled, VmemStorage::newBox and every public constructor that funnels through it — ConcurrentHeapRB::defaultcap, ConcurrentHeapRB::fromVec, From, etc. bit-copies the input buffer into a freshly mmap'd region with ptr::copynonoverlapping, then lets the source Box drop...