fdisk: sgi: abort on HDIO_GETGEO failure

fdisk: sgi: abort on HDIO_GETGEO failure

When the HDIO_GETGEO ioctl fails on non IRIX/MIPS platforms (ie: inappropriate
ioctl for device) the variables that describe the geometry are compromissed.
One clear example is a division by 0 bug because the cylinder size is 0 is
verify_sgi():

$> fdisk sgi.img
Welcome to fdisk (util-linux 2.21.392-4b1c).

...

Command (m for help): x

Expert command (m for help): g
Building a new SGI disklabel.
Partition 11 of type SGI volume and of size 7.9 MiB is set
Floating point exception

Fix this by simply exiting the program instead of leaving it in a vulnerable
state.

Signed-off-by: Davidlohr Bueso <dave@gnu.org>
Signed-off-by: Karel Zak <kzak@redhat.com>