Fix an infinite loop when the jids array is empty
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This merge request is automatically copied from GitHub master pull request https://github.com/nordugrid/arc/pull/15 orginally made by @dvandok
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The jobs_info routine matches the known jobs with those in
the batch system, but commit 91d2e601 introduces a bug
where the index in the jids array assumes that there is at
least one job.
Since the routine should return an empty hash in case jids is empty, get this done as soon as possible before doing any other work.