libfdisk: fix SIGFPE

 #0 recount_geometry at libfdisk/src/alignment.c:143
 #1 fdisk_discover_geometry at libfdisk/src/alignment.c:205
 #2 fdisk_context_assign_device at libfdisk/src/context.c:173
 #3 print_partition_table_from_option at fdisks/fdisk.c:924

References: https://bugzilla.redhat.com/show_bug.cgi?id=1005566
Signed-off-by: Karel Zak <kzak@redhat.com>