Treat a 'current' value of 0 as unlimited as a failsfe.

When limiting I/O, a value of 0 makes no sense as a limit. No progress
can be made. Trade the possibility that someone might be doing
something clever to achieve ultra-low I/O limits vs the damage of not
ever making progress on an I/O in favor of making progress. Now the
machine won't be useless if this accidentally gets requested.

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