amcheck: In verify_heapam, allows tuples with xmin 0.

Commit e88754a196 caused that case
to be reported as corruption, but Peter Geoghegan pointed out that
it can legitimately happen in the case of a speculative insertion
that aborts, so we'd better not flag it as corruption after all.

Back-patch to v14, like the commit that introduced the issue.

Discussion: http://postgr.es/m/CAH2-WzmEabzcPTxSY-NXKH6Qt3FkAPYHGQSe2PtvGgj17ZQkCw@mail.gmail.com

contrib/amcheck/verify_heapam.c

@@ -739,8 +739,7 @@ check_tuple_visibility(HeapCheckContext *ctx)
 	switch (get_xid_status(xmin, ctx, &xmin_status))
 	{
 		case XID_INVALID:
-			report_corruption(ctx,
-							  pstrdup("xmin is invalid"));
+			/* Could be the result of a speculative insertion that aborted. */
 			return false;
 		case XID_BOUNDS_OK:
 			break;