winpgntc: fix mishandling of named-pipe errors.

If named_pipe_agent_gotdata was called with an error or EOF status, it
would call agent_cancel_query(pq), but then accidentally fall through
to the non-error handler which would dereference pq. I meant to return
early in that situation, and Coverity spotted that I'd left out the
early return statement.

windows/winpgntc.c

@@ -195,6 +195,7 @@ static size_t named_pipe_agent_gotdata(
     if (err || len == 0) {
         pq->callback(pq->callback_ctx, NULL, 0);
         agent_cancel_query(pq);
+        return 0;
     }
 
     int status = named_pipe_agent_accumulate_response(pq->response, data, len);