libsmb: Use a direct struct initializer

libsmb: Use a direct struct initializer

There's a few ways to initialize a DATA_BLOB to NULL. There's the
variable data_blob_null, there's ZERO_STRUCTP, and the explicit
initializer. And there's the data_blob() macro which calls into
data_blob_talloc_named. You have to look at that routine to see that
this is nothing more than a ZERO_STRUCT() and not calling into
talloc. Choose the more direct way.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>