BUG #13487: GetBufferFromRing code bug

The following bug has been logged on the website:

Bug reference:      13487
Logged by:          will
Email address:      jingwei_5107@qq.com
PostgreSQL version: 9.4.4
Operating system:   It does not matter
Description:

In GetBufferFromRing method(/src/backend/storage/buffer/freelist.c),I think
the following code:
506     buf = &BufferDescriptors[bufnum - 1];
507    LockBufHdr(buf);
508    if (buf->refcount == 0 && buf->usage_count <= 1)
509    {
510        strategy->current_was_in_ring = true;
511        return buf;
512    }
513    UnlockBufHdr(buf);

should be changed to(Adding line 510):

506     buf = &BufferDescriptors[bufnum - 1];
507    LockBufHdr(buf);
508    if (buf->refcount == 0 && buf->usage_count <= 1)
509    {
510             UnlockBufHdr(buf);// add this line
511        strategy->current_was_in_ring = true;
512        return buf;
513    }
514    UnlockBufHdr(buf);

It should Unlock the buf before returning buf, after exclusively accessing
the buf's refcount&usage_count member.

Am I right? Any reply is appreciated if I'm wrong.