libfdisk: again improve freespace code

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: (dos) fix log.partition delete

 - deallocate sector buffer and deincrement part_max when delete last
   logical partition
 - zap obsolete EBR on begin of the extended partition when no logical
   partition defined

Signed-off-by: Karel Zak <kzak@redhat.com>