Quell Coverity for diskinfo(8)

• CID 1198994: Don't run the speed disk on a disk with no sectors
• CID 1011442: Don't call close(2) if open(2) fails
• CID 1011161: Use snprintf instead of sprintf
• CID 1009825: Check the return value of lseek

Reported by: Coverity
CID: 1198994 1011442 1011161 1009825
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp