hs: Descriptor parsing legacy cert can lead to assert
This is the legacy key cert part of #41259 as they are two different issues and different TROVE.
```c
tok = find_opt_by_keyword(tokens, R3_INTRO_LEGACY_KEY_CERT);
if (!tok) {
log_warn(LD_REND, "Introduction point legacy key cert is missing");
goto err;
}
tor_assert(tok->object_body);
```
This is now TROVE-2026-015
issue