rpc_server: Fix some uses of tevent_req_nomem

rpc_server: Fix some uses of tevent_req_nomem

tevent_req_nomem is to be used in a sequence of async actions where we
have one main request. This is a completely independent loop without one
central tevent_req. tevent_req_nomem is used as a simple way to signal
an out of memory condition to the main request representing the async
sequence. If we don't have such a tevent_req, we need to directly check
for NULL.

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