avoid arithmetic overflow in the examples/file/readspeed.c and writespeed.c

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1916 bbd45198-f89e-11dd-88c7-29a3b14d5316

examples/file/readspeed.c

@@ -55,7 +55,7 @@ void readspeed(const char* filename, int block_size)
 	rt_free(buff_ptr);
 
     /* calculate read speed */
-    rt_kprintf("File read speed: %d byte/s\n", total_length*RT_TICK_PER_SECOND/tick);
+    rt_kprintf("File read speed: %d byte/s\n", total_length /tick * RT_TICK_PER_SECOND);
 }
 
 #ifdef RT_USING_FINSH

examples/file/writespeed.c

@@ -63,7 +63,7 @@ void writespeed(const char* filename, int total_length, int block_size)
 	rt_free(buff_ptr);
 
     /* calculate write speed */
-    rt_kprintf("File write speed: %d byte/s\n", total_length*RT_TICK_PER_SECOND/tick);
+    rt_kprintf("File write speed: %d byte/s\n", total_length / tick * RT_TICK_PER_SECOND);
 }
 
 #ifdef RT_USING_FINSH