libfdisk: (dos) fix logical partitions add/delete code

If you delete logical partition and then create a new one than fdisk
(and cfdisk) might write EBR to the first sector on the device. That's
wrong of course; because you will lost MBR (primary partitions).

(Probably introduced by commit bcddbe96882b88d53b6bc0495e7322c0820a5122
 where code completely clears EBR stuff.)

Reported-by: Michał Górny <mgorny@gentoo.org>
Signed-off-by: Karel Zak <kzak@redhat.com>