Add a sanity check for bad StatFS results.

git-annex FTBFS on s390, mips, powerpc, sparc. That StatFS code is failing
on all of them. At least on s390, the failure appears as:

Just (FileSystemStats {fsStatBlockSize = 4096, fsStatBlockCount = 0,
fsStatByteCount = 0, fsStatBytesFree = 0, fsStatBytesAvailable = 0,
fsStatBytesUsed = 0})

While I don't understand why this is happening, or how to fix it,
bandaid over it by checking for obviously bad values and returning Nothing.
That disables disk free space checking, but at least git-annex will work.

Upstream bug: http://code.google.com/p/xmobar/issues/detail?id=70

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