libfdisk: (dos) fix logical partitions errors handling

libfdisk: (dos) fix logical partitions errors handling

The current code in add_partition() de-increment number of partitions
on error when logical partition requested and add_logical() does the
same. The result is mess in partitions array.

The another fixed issue is that add_logical() is called if all primary
partitions are already used although 'pa->start' (the template) is
explicitly defined outside the extended partition. The right behavior
is to end with error message.

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