Null pointer problem caused coredump

At this position in the code, an attempt was made to write the value of `job->jobId`.
However, the variable job was initially defined as NULL, and there was no valid definition before it was output here, thus resulting in a null pointer.
https://github.com/citusdata/pg_cron/blob/237c5250c8027bb6d0612a111b2c72537d7e1824/src/job_metadata.c#L928
https://github.com/citusdata/pg_cron/blob/237c5250c8027bb6d0612a111b2c72537d7e1824/src/job_metadata.c#L963

Set the parameter `cron.enable_superuser_jobs` to false and set the owner of the newly created task to the user with superuser permissions to trigger coredump.