Fix ubsan null pointer passed as argument 2

Fix ubsan null pointer passed as argument 2

Fix ubsan warning null pointer passed as argument 2 when the source
pointer is NULL.  The calls to memcpy are now guarded by an
if (len > 0)

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Gary Lockyer <gary@samba.org>
Autobuild-Date(master): Mon May 27 01:29:48 UTC 2019 on sn-devel-184