fix crash bug in pidl generated client code, this

fix crash bug in pidl generated client code, this
could have happend with [in,out,unique] pointers
when the clients sends a valid pointer, but the server
reponse with a NULL pointer (as samba-3.0.26a do for some calls).

I've tested with midl to see how windows handles this situation
and also the reverse case where the client sends NULL and
the server reposnse with non-NULL.

It appears that midl generated code just ignores this
and only copies the result if both pointers are non-NULL.

Note: this is just cosmetic for the 3.0.x tree, as
rpccli_wkssvc_NetWkstaEnumUsers and rpccli_wkssvc_NetWkstaTransportEnum
are not used.

metze