libdsk: reset geometry if autoprobe isn't sane So that weirdness from any misfiring auto-geometry heuristic in LibDsk doesn't cause trouble later. (We were overwriting the most common parameters, but not all of them.) (Triggered by Debian bug #1079619, where a spurious dg_sidedness caused trouble after LibDsk's Opus Discovery heuristic misfired -- that heuristic has been improved in libdsk 1.5.20, but this change provides defence in depth.)
