examples/10-at-a-time: fix possible skipped final transfers

Prior to this change if curl_multi_perform returned 0 running handles
and then all remaining transfers were added, then the perform loop would
end immediately without performing those transfers.

Reported-by: Mikhail Kuznetsov

Fixes https://github.com/curl/curl/issues/9953
Closes https://github.com/curl/curl/pull/9954

docs/examples/10-at-a-time.c

@@ -95,13 +95,14 @@ static size_t write_cb(char *data, size_t n, size_t l, void *userp)
   return n*l;
 }
 
-static void add_transfer(CURLM *cm, int i)
+static void add_transfer(CURLM *cm, int i, int *left)
 {
   CURL *eh = curl_easy_init();
   curl_easy_setopt(eh, CURLOPT_WRITEFUNCTION, write_cb);
   curl_easy_setopt(eh, CURLOPT_URL, urls[i]);
   curl_easy_setopt(eh, CURLOPT_PRIVATE, urls[i]);
   curl_multi_add_handle(cm, eh);
+  (*left)++;
 }
 
 int main(void)
@@ -110,7 +111,7 @@ int main(void)
   CURLMsg *msg;
   unsigned int transfers = 0;
   int msgs_left = -1;
-  int still_alive = 1;
+  int left = 0;
 
   curl_global_init(CURL_GLOBAL_ALL);
   cm = curl_multi_init();
@@ -118,10 +119,12 @@ int main(void)
   /* Limit the amount of simultaneous connections curl should allow: */
   curl_multi_setopt(cm, CURLMOPT_MAXCONNECTS, (long)MAX_PARALLEL);
 
-  for(transfers = 0; transfers < MAX_PARALLEL; transfers++)
-    add_transfer(cm, transfers);
+  for(transfers = 0; transfers < MAX_PARALLEL && transfers < NUM_URLS;
+      transfers++)
+    add_transfer(cm, transfers, &left);
 
   do {
+    int still_alive = 1;
     curl_multi_perform(cm, &still_alive);
 
     while((msg = curl_multi_info_read(cm, &msgs_left))) {
@@ -133,17 +136,18 @@ int main(void)
                 msg->data.result, curl_easy_strerror(msg->data.result), url);
         curl_multi_remove_handle(cm, e);
         curl_easy_cleanup(e);
+        left--;
       }
       else {
         fprintf(stderr, "E: CURLMsg (%d)\n", msg->msg);
       }
       if(transfers < NUM_URLS)
-        add_transfer(cm, transfers++);
+        add_transfer(cm, transfers++, &left);
     }
-    if(still_alive)
+    if(left)
       curl_multi_wait(cm, NULL, 0, 1000, NULL);
 
-  } while(still_alive || (transfers < NUM_URLS));
+  } while(left);
 
   curl_multi_cleanup(cm);
   curl_global_cleanup();