Add _bt_binsrch() scantid assertion to nbtree.

Assert that _bt_binsrch() binary searches with scantid set in insertion
scankey cannot be performed on leaf pages.  Leaf-level binary searches
where scantid is set must use _bt_binsrch_insert() instead.

_bt_binsrch_insert() is likely to have additional responsibilities in
the future, such as searching within GIN-style posting lists using
scantid.  It seems like a good idea to tighten things up now.

src/backend/access/nbtree/nbtsearch.c

@@ -347,12 +347,14 @@ _bt_binsrch(Relation rel,
 	int32		result,
 				cmpval;
 
-	/* Requesting nextkey semantics while using scantid seems nonsensical */
-	Assert(!key->nextkey || key->scantid == NULL);
-
 	page = BufferGetPage(buf);
 	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
 
+	/* Requesting nextkey semantics while using scantid seems nonsensical */
+	Assert(!key->nextkey || key->scantid == NULL);
+	/* scantid-set callers must use _bt_binsrch_insert() on leaf pages */
+	Assert(!P_ISLEAF(opaque) || key->scantid == NULL);
+
 	low = P_FIRSTDATAKEY(opaque);
 	high = PageGetMaxOffsetNumber(page);