tool_operate: repair --rate

Regression from a55256cfb242 (7.87.0)
Reported-by: highmtworks on github
Fixes #10357
Closes #10358

src/tool_operate.c

@@ -2421,6 +2421,7 @@ static CURLcode serial_transfers(struct GlobalConfig *global,
     bool retry;
     long delay_ms;
     bool bailout = FALSE;
+    struct timeval start;
     result = pre_transfer(global, per);
     if(result)
       break;
@@ -2430,7 +2431,7 @@ static CURLcode serial_transfers(struct GlobalConfig *global,
       if(result)
         break;
     }
-
+    start = tvnow();
 #ifdef CURLDEBUG
     if(global->test_event_based)
       result = curl_easy_perform_ev(per->curl);
@@ -2462,7 +2463,7 @@ static CURLcode serial_transfers(struct GlobalConfig *global,
     if(per && global->ms_per_transfer) {
       /* how long time did the most recent transfer take in number of
          milliseconds */
-      long milli = tvdiff(tvnow(), per->start);
+      long milli = tvdiff(tvnow(), start);
       if(milli < global->ms_per_transfer) {
         notef(global, "Transfer took %ld ms, waits %ldms as set by --rate\n",
               milli, global->ms_per_transfer - milli);