fix cat print buffer no \0

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1176 bbd45198-f89e-11dd-88c7-29a3b14d5316

components/dfs/src/dfs_file.c

@@ -516,7 +516,7 @@ void cat(const char* filename)
 	do
 	{
 		rt_memset(buffer, 0, sizeof(buffer));
-		length = dfs_file_read(&fd, buffer, 81);
+		length = dfs_file_read(&fd, buffer, sizeof(buffer)-1 );
 		if (length > 0)
 		{
 			rt_kprintf("%s", buffer);