equip SMARTData with a validity check

this fixes vbox drives getting reported as possibly breaking when indeed they simply lack any amount of smart data. currently I have no evidence that this is a more wide spread issue beyond vbox so the validity condition is fairly constrained.

this also sees a slight refactor of smartctl which previously had error some weak failure handling, by moving that outside the kauth-dependent code we can actually test this though and smartmonitor is easily retro fitted for validity handling anyway