blockdev: Fix error message for invalid -drive CHS

When cyls, heads or secs are out of range, the error message prints
buf, which points to the value of option "if".  Bogus, may even be
null.  Drop that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
