Fix WAL-logging of B-tree "unlink halfdead page" operation.

There was some confusion on how to record the case that the operation
unlinks the last non-leaf page in the branch being deleted.
_bt_unlink_halfdead_page set the "topdead" field in the WAL record to
the leaf page, but the redo routine assumed that it would be an invalid
block number in that case. This commit fixes _bt_unlink_halfdead_page to
do what the redo routine expected.

This code is new in 9.4, so backpatch there.

src/backend/access/nbtree/nbtpage.c

@@ -1567,7 +1567,6 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, bool *rightsib_empty)
 	int			targetlevel;
 	ItemPointer leafhikey;
 	BlockNumber nextchild;
-	BlockNumber topblkno;
 
 	page = BufferGetPage(leafbuf);
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
@@ -1591,11 +1590,10 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, bool *rightsib_empty)
 	 */
 	if (ItemPointerIsValid(leafhikey))
 	{
-		topblkno = ItemPointerGetBlockNumber(leafhikey);
-		target = topblkno;
+		target = ItemPointerGetBlockNumber(leafhikey);
 
 		/* fetch the block number of the topmost parent's left sibling */
-		buf = _bt_getbuf(rel, topblkno, BT_READ);
+		buf = _bt_getbuf(rel, target, BT_READ);
 		page = BufferGetPage(buf);
 		opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 		leftsib = opaque->btpo_prev;
@@ -1609,7 +1607,6 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, bool *rightsib_empty)
 	}
 	else
 	{
-		topblkno = InvalidBlockNumber;
 		target = leafblkno;
 
 		buf = leafbuf;
@@ -1694,9 +1691,11 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, bool *rightsib_empty)
 			elog(ERROR, "half-dead page changed status unexpectedly in block %u of index \"%s\"",
 				 target, RelationGetRelationName(rel));
 
-		/* remember the next child down in the branch. */
+		/* remember the next non-leaf child down in the branch. */
 		itemid = PageGetItemId(page, P_FIRSTDATAKEY(opaque));
 		nextchild = ItemPointerGetBlockNumber(&((IndexTuple) PageGetItem(page, itemid))->t_tid);
+		if (nextchild == leafblkno)
+			nextchild = InvalidBlockNumber;
 	}
 
 	/*
@@ -1782,7 +1781,7 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, bool *rightsib_empty)
 	 */
 	if (target != leafblkno)
 	{
-		if (nextchild == leafblkno)
+		if (nextchild == InvalidBlockNumber)
 			ItemPointerSetInvalid(leafhikey);
 		else
 			ItemPointerSet(leafhikey, nextchild, P_HIKEY);