fix a potential bug with readlink(2)

According to linux manual READLINK(2)

   readlink() places the contents of the symbolic link pathname in the buffer
   buf,  which has size bufsiz. readlink() does not append a null byte to buf.
   It will truncate the contents (to a length of bufsiz characters), in case
   the buffer is too small to hold all of the contents.

there is a situation that readlink returns bufsize when the path is long enough
, and in that situation we would write \0 to the invalid address buf[bufsize].
obviously it is not allowed.