sysbench randomly stops with error: raise Exception(f"sysbench failed with {proc.poll()}"); Exception: sysbench failed with -11

## Steps to reproduce
Deploy PG16 r798 to Raspberry Pi5 ARM + sysbench r186 and prepare/run test in scale=1,thread=4.
Fetch results using `curl 10.239.193.235:8088` (where 10.239.193.235 is sysbench unit ip).

## Expected behavior
The performance test went fine until user stop it.

## Actual behavior
The performance test has stopped unexpectedly with error:
```
Jun 24 10:10:57 juju-842b82-2 systemd[1]: Started Service for controlling sysbench.
Jun 24 11:09:32 juju-842b82-2 sysbench_svc.py[22638]: sysbench STDOUT:
Jun 24 11:09:32 juju-842b82-2 sysbench_svc.py[22638]: sysbench STDERR:
Jun 24 11:09:33 juju-842b82-2 sysbench_svc.py[22638]: Traceback (most recent call last):
Jun 24 11:09:33 juju-842b82-2 sysbench_svc.py[22638]: File "/usr/bin/sysbench_svc.py", line 190, in
Jun 24 11:09:33 juju-842b82-2 sysbench_svc.py[22638]: main(args)
Jun 24 11:09:33 juju-842b82-2 sysbench_svc.py[22638]: File "/usr/bin/sysbench_svc.py", line 160, in main
Jun 24 11:09:33 juju-842b82-2 sysbench_svc.py[22638]: raise Exception(f"sysbench failed with {proc.poll()}")
Jun 24 11:09:33 juju-842b82-2 sysbench_svc.py[22638]: Exception: sysbench failed with -11
Jun 24 11:09:33 juju-842b82-2 systemd[1]: sysbench.service: Main process exited, code=exited, status=1/FAILURE
Jun 24 11:09:33 juju-842b82-2 systemd[1]: sysbench.service: Failed with result 'exit-code'.
Jun 24 11:09:33 juju-842b82-2 systemd[1]: sysbench.service: Consumed 31min 21.421s CPU time.
```


## Versions

Operating system: 24.04 LTS latest
Juju CLI: 3.6.7
Juju agent: 3.6.7
Charm revision: see above
LXD: 6.4-1c216d4 34287 latest/stable

Start with /usr/bin/sysbench_svc.py around line 160, where the reported exception is raised, and reproduce the scale=1, thread=4 run on the stated Raspberry Pi 5 ARM and PG16 environment. Investigate what exit code -11 represents and verify that the performance test does not stop unexpectedly during the same workload.

Written by the indexing model from the issue text.