ntp: don't stop online burst when sending fails

Don't stop online burst for unreachable sources until sending succeeds.
This is mainly useful with iburst when chronyd is started before the
network is configured.

ntp_core.c

@@ -891,6 +891,10 @@ transmit_timeout(void *arg)
 
   switch (inst->opmode) {
     case MD_BURST_WAS_ONLINE:
+      /* When not reachable, don't stop online burst until sending succeeds */
+      if (!sent && !SRC_IsReachable(inst->source))
+        break;
+      /* Fall through */
     case MD_BURST_WAS_OFFLINE:
       --inst->burst_total_samples_to_go;
       break;