heap_lock_updated_tuple_rec can leak a buffer refcount

It seems to me that heap_lock_updated_tuple_rec can lead to a buffer
refcount leak while locking an updated tuple by an aborted
transaction. In commit - 5c609a74, we have added the code to deal
with aborted transactions as below:

heap_lock_updated_tuple_rec()
{
..

if (PageIsAllVisible(BufferGetPage(buf)))
visibilitymap_pin(rel, block, &vmbuffer);
else
vmbuffer = InvalidBuffer;

LockBuffer(buf, BUFFER_LOCK_EXCLUSIVE);
..
-------------------------- below code is added by commit -5c609a74 -----------
if (TransactionIdDidAbort(HeapTupleHeaderGetXmin(mytup.t_data)))
{
UnlockReleaseBuffer(buf);
return HeapTupleMayBeUpdated;
}
-------------------------------------------------------------

I think the above code forgets to deal with vmbuffer and can lead to a
leak of the same. Attached patch ensures that it deals with vmbuffer
when required.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com