This bug was introduced by https://github.com/redis/redis/pull/14130
found by @oranagra
### Summary
Because `s->cgroup_ref` is created at runtime the first time a consumer
group is linked with a message, but it is not released when all
references are removed.
However, after `debug reload` or restart, if the PEL is empty (meaning
no consumer group is referencing any message), `s->cgroup_ref` will not
be recreated.
As a result, when executing XADD or XTRIM with `ACKED` option and
checking whether a message that is being read but has not been ACKed can
be deleted, the cgroup_ref being NULL will cause a crash.
### Code Path
```
xaddCommand -> streamTrim -> streamEntryIsReferenced
```
### Solution
Check if `s->cgroup_ref` is NULL in streamEntryIsReferenced().
3614cea1ad