Werk #15849: Filesystem checks: Avoid crash if device reports negative free space

A negative value for the free space is of course nonsense. However, if a device reported this, Checkmk might have crashed with

ValueError("Cannot render negative timespan")

when attempting to compute the time left until the filesystem is full. Now, we instead report a value of 0 seconds in this case, st. users receive informative notifications about the problem.

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