revert: Remove meaningless assignments in nbtree code

Reverts commit 05684c8255.

Reported-by: Tom Lane

Discussion: https://postgr.es/m/404.1576770942@sss.pgh.pa.us

Backpatch-through: master

src/backend/access/nbtree/nbtpage.c

@@ -1600,6 +1600,7 @@ _bt_mark_page_halfdead(Relation rel, Buffer leafbuf, BTStack stack)
 	 * fail in the field, though.
 	 */
 	page = BufferGetPage(topparent);
+	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
 #ifdef USE_ASSERT_CHECKING
 	itemid = PageGetItemId(page, topoff);

src/backend/access/nbtree/nbtxlog.c

@@ -590,6 +590,8 @@ btree_xlog_mark_page_halfdead(uint8 info, XLogReaderState *record)
 		BlockNumber rightsib;
 
 		page = (Page) BufferGetPage(buffer);
+		pageop = (BTPageOpaque) PageGetSpecialPointer(page);
+
 		poffset = xlrec->poffset;
 
 		nextoffset = OffsetNumberNext(poffset);