Fix potential SSI hazard in heap_update().

Commit 6f38d4da failed to heed a warning about the stability of the
value pointed to by "otid".  The caller is allowed to pass in a pointer to
newtup->t_self, which will be updated during the execution of the
function.  Instead, the SSI check should use the value we copy into
oldtup.t_self near the top of the function.

Not a live bug, because newtup->t_self doesn't really get updated until
a bit later, but it was confusing and broke the rule established by the
comment.

Back-patch to 13.
Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/2689164.1618160085%40sss.pgh.pa.us

src/backend/access/heap/heapam.c

@@ -3900,7 +3900,8 @@ l2:
 	 * will include checking the relation level, there is no benefit to a
 	 * separate check for the new tuple.
 	 */
-	CheckForSerializableConflictIn(relation, otid, BufferGetBlockNumber(buffer));
+	CheckForSerializableConflictIn(relation, &oldtup.t_self,
+								   BufferGetBlockNumber(buffer));
 
 	/*
 	 * At this point newbuf and buffer are both pinned and locked, and newbuf