BUG: SIGABRT in quantiles_sketch::deserialize(): dereferencing empty std::optional (libc++ verbose_abort)

[MaheshGPai]

Some deserialization paths use std::optional<T> tmp; as if it were “allocated but uninitialized storage” and then pass &*tmp to serde::deserialize(...). When tmp is disengaged, operator* is undefined behavior; with libc++ hardening enabled it triggers an assertion and aborts the process (SIGABRT).

This results in process termination instead of a normal error (exception) when deserializing malformed/short buffers (and potentially other edge cases).

Expected behavior

Deserialization should either:

• succeed, or
• throw/return a normal error (e.g. std::invalid_argument, std::runtime_error)

It should not abort the process.

Actual behavior

With libc++ hardening enabled, the process aborts with:

• optional operator* called on a disengaged value
• Abort trap: 6 / SIGABRT

The abort points to std::optional<T>::operator* used on an empty optional.