[HUST CSE] Fix: 字符数组str越界

[7YZ7]

为什么提交这份PR (why to submit this PR) 在board/TencentOS_Tiny_EVB_STM32WL/KEIL/lorawan_pm25/App/mqtt_iot_explorer_esp8266.c这个文件中，第68行创建的字符数组str大小只有16，后续后续148sprintf的字符串超出了16的大小，会发生数组越界，出现问题

你的解决方案是什么 (what is your solution) 将str字符数组开大空间，字符串"PM2.5:%4d ug/m3"包含结束符的长度为17，因此开辟32的空间则可以保证不越界

在什么测试环境下测试通过 (what is the test environment) 只增加了str的空间，因此测试一定会通过

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[tencent-adm]

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