Not sure if I have understood your concerns correctly:

There is no error handling really, the function just returns, if any `goto cleanup;` is reached:
When _jumped_ to this label `rettv->vval.v_number` is `0` and `argv` is `NULL`, always. A call to `shell_free_argv` returns immediatley, without freeing anything. So the cleanup does nothing.

The only purpose of this label is to free `argv` if `job_start` failed, which could be done more consitent in the job module.