Btrfs-progs: we need to have the string null terminated

Btrfs-progs: we need to have the string null terminated

Bug:
-------------
btrfs subvolume list / -a
ID 258 gen 4226 top level 384 path media/smbshare
::
btrfs subvolume list /home -a
ID 258 gen 4226 top level 5 path <FS_TREE>/__active/media/smbshare4.snap

In the first command's output, this path is printed correctly, however
in the second output it has "4.snap" appended, similar to the names of
the snapshots I made 22 hours ago.
------------

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reported-by: Brendan Hide <brendan@swiftspirit.co.za>