fdisk: fix segfault on bsd label

fdisk: fix segfault on bsd label

Commit 8db8295d824cd0c8cba9385e4635d6e311d69d3f added a regression that causes the program to crash when
touching the partition structure (pte) for BSD/OSF labels. Since DOS has its own initialization function,
allow BSD labels to use it as well.

Steps to reproduce:

$> fdisk bsd.img (obtained from blkid regression test files)
Command (m for help): p
...
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Segmentation fault

Signed-off-by: Davidlohr Bueso <dave@gnu.org>