fix(netdev): 修复无网卡时 ping 命令空指针崩溃

netdev_cmd_ping 中 netdev 回退到 netdev_default 后未判空，
QEMU 无网卡环境下执行 ping 直接解引用 NULL 崩溃（data abort）。
回退后增加空指针检查，提示无可用网卡并返回错误。

Author: Huoyanlifusu

components/net/netdev/src/netdev.c

@@ -1340,6 +1340,11 @@ int netdev_cmd_ping(char* target_name, char *netdev_name, rt_uint32_t times, rt_
     if (netdev == RT_NULL)
     {
         netdev = netdev_default;
+        if (netdev == RT_NULL)
+        {
+            rt_kprintf("ping: not found default netif, please specify netdev name.\n");
+            return -RT_ERROR;
+        }
         rt_kprintf("ping: not found specified netif, using default netdev %s.\n", netdev->name);
     }
 
@@ -1412,6 +1417,12 @@ int netdev_cmd_ping(char* target_name, char *netdev_name, rt_uint32_t times, rt_
         rt_thread_delay(delay_tick);
     }
 
+    if (times == 0)
+    {
+        rt_kprintf("ping: send times is zero, no ping statistics.\n");
+        return -RT_ERROR;
+    }
+
     /* print ping statistics */
     loss = (uint32_t)((1 - ((float)received) / index) * 100);
     avg_time = (uint32_t)(avg_time / received);