This fixes several Coverity-detected errors in
nvmecontrol. While in here, a couple additional errors with shift/mask
confusion that were not diagnosed by Coverity are also fixed.
CIDs addressed: 1040299, 1040300, 1403972, 1403973, 1403985,
1403988, 1403990, 1404374, 1404427, 1404469, 1404510, 1404534, 1418118
CID 1403657 (resource leak of shared library handle) was marked "intentional" in the Coverity scan database.