blkid: suppress Coverity warning

blkid: suppress Coverity warning

The getopt() function will never let optarg be NULL (at least without
using the GNU double-colon extension, which we don't use because it's
not portable), so don't bother checking for that case.  It's harmless,
but it triggers a Coverity warning elsewhere, since it thinks optarg
could in fact be NULL.

Addresses-Coverity-Id: #1049156

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>