Do not happily NBD "map" non-existing images, do not try to benchmark them too

src/fio_cluster.cpp

@@ -247,6 +247,12 @@ static int sec_setup(struct thread_data *td)
             vitastor_c_uring_wait_events(bsd->cli);
         }
         td->files[0]->real_file_size = vitastor_c_inode_get_size(bsd->watch);
+        if (!vitastor_c_inode_get_num(bsd->watch) ||
+            !td->files[0]->real_file_size)
+        {
+            td_verror(td, EINVAL, "image does not exist");
+            return 1;
+        }
     }
 
     bsd->trace = o->trace ? true : false;

src/nbd_proxy.cpp

@@ -190,6 +190,12 @@ public:
             }
             watch = cli->st_cli.watch_inode(image_name);
             device_size = watch->cfg.size;
+            if (!watch->cfg.num || !device_size)
+            {
+                // Image does not exist
+                fprintf(stderr, "Image %s does not exist\n", image_name.c_str());
+                exit(1);
+            }
         }
         // Initialize NBD
         int sockfd[2];