libmount: FS id and parent ID could be zero

It seems that linux 3.14 is able to produce things like:

  19 0 8:3 / / rw,relatime - ext4 /dev/sda3 rw,data=ordered
     ^

Reported-by: Mantas Mikulėnas <grawity@gmail.com>
Signed-off-by: Karel Zak <kzak@redhat.com>