tdb: Fix CID 1034959 Uninitialized scalar variable

log_ctx.log_private was used uninitialized. Not a real bug here,
as tdb_log does not access it, but tdb_open_ex still moves around
uninitialized data. So this would show up in valgrind as well.

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