Double free / use-after-free in `Consumer::skip` and `Consumer::clear` when an element's `Drop` panics

`Consumer::skip()` and `Consumer::clear()` are not panic-safe. They drop the
consumed elements in place and only afterwards call `advance_read_index()` to move
the ring buffer's read index past them. If an element's `Drop` panics mid-loop,
`advance_read_index()` is never reached, so the read index still points at the
already-dropped elements. When the ring buffer is later dropped, its destructor
re-visits those slots and drops the same elements a second time — a double free
(CWE-415) / use-after-free (CWE-416) reachable from safe Rust, confirmed under
AddressSanitizer.

`Consumer::clear()` delegates to `Consumer::skip(self.len())`, so both share the
same root cause and the same fix.

## Mitigation

Update to 0.5.2 or later (fixed in agerasev/ringbuf#60).