examples/kernel: fix warnings issued by Clang

Fix some "unused variable" and "used before set" warnings.

examples/kernel/event_simple.c

@@ -7,6 +7,7 @@
  * 一个线程定时发送事件 (事件5)
  */
 #include <rtthread.h>
+#include <time.h>
 #include "tc_comm.h"
 
 /* 指向线程控制块的指针 */

examples/kernel/mutex_simple.c

@@ -60,6 +60,10 @@ static void thread3_entry(void* parameter)
     while (1)
     {
         result = rt_mutex_take(mutex, RT_WAITING_FOREVER);
+        if (result != RT_EOK)
+        {
+            tc_stat(TC_STAT_END | TC_STAT_FAILED);
+        }
         result = rt_mutex_take(mutex, RT_WAITING_FOREVER);
         if (result != RT_EOK)
         {

examples/kernel/semaphore_buffer_worker.c

@@ -178,8 +178,14 @@ static void worker_entry(void* parameter)
 
         /* 持有信号量 */
         rt_sem_take(sem, RT_WAITING_FOREVER);
+
         /* 把数据放到环形buffer中 */
         result = rb_put(&working_rb, &data_buffer[0], BUFFER_ITEM);
+        if (result == RT_FALSE)
+        {
+            rt_kprintf("put error\n");
+        }
+
         /* 释放信号量 */
         rt_sem_release(sem);
 

examples/kernel/semaphore_producer_consumer.c

@@ -59,6 +59,7 @@ void consumer_thread_entry(void* parameter)
     /* 第n个线程，由入口参数传进来 */
     no = (rt_uint32_t)parameter;
 
+    sum = 0;
     while(1)
     {
         /* 获取一个满位 */