talloc: Fix alignment issues for casting pointers

warning: cast from 'char *' to 'struct talloc_chunk *' increases required
alignment from 1 to 8

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>

Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Tue Mar 19 12:38:50 UTC 2019 on sn-devel-144
