Fix check for 0-size CDOs

reported no-memory-registration errors for almost all CDOs now due to a
typo in dccf6ec0

maestro/pool_client.c

@@ -808,7 +808,7 @@ mstro_pc__handle_transfer_completed(Mstro__Pool__MstroMsg *msg)
       goto DONE;
     }
     int64_t realsize = *(int64_t*)size;
-    if(realsize==-1 && realsize!=0) {
+    if(realsize==-1 || realsize==0) {
       s=MSTRO_OK; // 0-size CDO or type-0 CDO
       DEBUG("Source CDO empty, doing NULL transfer\n");
       s=MSTRO_OK;