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>