luks2: Make sure all fields of output argument in luks2_parse_digest() are written to

luks2: Make sure all fields of output argument in luks2_parse_digest() are written to

We should assume that the output argument "out" is uninitialized and could
have random data. So, make sure to initialize the segments and keyslots bit
fields because potentially not all bits of those fields are written to.
Otherwise, the digest could say it belongs to keyslots and segments that it
does not.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>