fix: audio_free_buffers() add a NULL check.

On the Zhaoxin processor, there is a probabilistic occurrence of null pointer/double-free issues with `audio_buffers[i].data`, so a NULL check needs to be added.
在兆芯处理器上，audio_buffers[i].data 概率性出现空指针/重复释放的情况，所以需要增加 NULL 判断。

Bug: https://pms.uniontech.com/bug-view-345713.html

Author: lichaofan2008

libcam/libcam_audio/audio.c

@@ -131,7 +131,13 @@ static void audio_free_buffers()
 
 	for(i = 0; i < AUDBUFF_NUM; ++i)
 	{
+		// 在兆芯处理器上，audio_buffers[i].data 概率性出现空指针/重复释放的情况，所以这里需要增加判断
+		if (audio_buffers[i].data == NULL) {
+			continue;
+		}
+
 		free(audio_buffers[i].data);
+		audio_buffers[i].data = NULL;
 	}
 
 	free(audio_buffers);