fix(netdev): 修复无网卡时 ping 命令空指针崩溃

[Huoyanlifusu]

拉取/合并请求描述：(PR description)

[

为什么提交这份PR (why to submit this PR)

Fixes #11744

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）。

你的解决方案是什么 (what is your solution)

回退后增加空指针检查，提示无可用网卡并返回错误。

请提供验证的bsp和config (provide the config and bsp)

• BSP:

bsp/qemu-vexpress-a9

• .config:

@@ -419,7 +420,15 @@ CONFIG_RT_USING_POSIX_MESSAGE_SEMAPHORE=y
 # Network
 #
 # CONFIG_RT_USING_SAL is not set
-# CONFIG_RT_USING_NETDEV is not set
+CONFIG_RT_USING_NETDEV=y
+CONFIG_NETDEV_USING_IFCONFIG=y
+CONFIG_NETDEV_USING_PING=y
+CONFIG_NETDEV_USING_NETSTAT=y
+CONFIG_NETDEV_USING_AUTO_DEFAULT=y
+# CONFIG_NETDEV_USING_LINK_STATUS_CALLBACK is not set
+# CONFIG_NETDEV_USING_IPV6 is not set
+CONFIG_NETDEV_IPV4=1
+CONFIG_NETDEV_IPV6=0
 # CONFIG_RT_USING_LWIP is not set
 # CONFIG_RT_USING_AT is not set
 # end of Network

• action:

WARNING: Image format was not specified for 'sd.bin' and probing guessed raw.
         Automatically detecting the format is dangerous for raw images, write operations on block 0 will be restricted.
         Specify the 'raw' format explicitly to remove the restrictions.
pulseaudio: set_sink_input_volume() failed
pulseaudio: Reason: Invalid argument
pulseaudio: set_sink_input_mute() failed
pulseaudio: Reason: Invalid argument

 \ | /
- RT -     Thread Operating System
 / | \     5.3.0 build Aug 26 2026 14:33:35
 2006 - 2024 Copyright by RT-Thread team
[I/SDIO] SD card capacity 65536 KB.
[I/SDIO] SD card switch to High Speed / SDR25 mode
[I/FileSystem] file system initialization done!

Hello RT-Thread!
msh />ping
ping
msh />ping 1.2.3.4
ping: not found default netif, please specify netdev name.
msh />ping 1.2.3.4 0
ping: not found default netif, please specify netdev name.
msh />ping 1        
ping: not found default netif, please specify netdev name.
msh />      

]

当前拉取/合并请求的状态 Intent for your PR

必须选择一项 Choose one (Mandatory):

• 本拉取/合并请求是一个草稿版本 This PR is for a code-review and is intended to get feedback
• 本拉取/合并请求是一个成熟版本 This PR is mature, and ready to be integrated into the repo

代码质量 Code Quality：

我在这个拉取/合并请求中已经考虑了 As part of this pull request, I've considered the following:

• 已经仔细查看过代码改动的对比 Already check the difference between PR and old code
• 代码风格正确，包括缩进空格，命名及其他风格 Style guide is adhered to, including spacing, naming and other styles
• 没有垃圾代码，代码尽量精简，不包含#if 0代码，不包含已经被注释了的代码 All redundant code is removed and cleaned up
• 所有变更均有原因及合理的，并且不会影响到其他软件组件代码或BSP All modifications are justified and not affect other components or BSP
• 对难懂代码均提供对应的注释 I've commented appropriately where code is tricky
• 代码是高质量的 Code in this PR is of high quality
• 已经使用clang-format 源码格式化工具确保格式符合RT-Thread代码规范 This PR has been formatted with clang-format and complies with RT-Thread code specification
• 如果是新增bsp, 已经添加ci检查到.github/ALL_BSP_COMPILE.json 详细请参考链接BSP自查

[CLAassistant]

All committers have signed the CLA.

[github-actions]

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为确保代码符合 RT-Thread 的编码规范，请在你的仓库中执行以下步骤运行代码格式化工作流（如果格式化CI运行失败）。
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Once completed, commits will be pushed to the fix/ping-netdev-null branch automatically, and the related Pull Request will be updated.

如有问题欢迎联系我们，再次感谢您的贡献！💐
If you have any questions, feel free to reach out. Thanks again for your contribution!

[github-actions]

📌 Code Review Assignment

🏷️ Tag: components

Reviewers: @Maihuanyi

Changed Files (Click to expand)

• components/net/netdev/src/netdev.c

---

📊 Current Review Status (Last Updated: 2026-08-26 14:21 CST)

• ⌛ @Maihuanyi Pending Review

---

📝 Review Instructions

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   Comment LGTM/lgtm after confirming approval

3. PR合并前需至少一位维护者确认
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