Fix broken Assert() introduced by 8e9a16ab8f

Don't assert MultiXactIdIsRunning if the multi came from a tuple that
had been share-locked and later copied over to the new cluster by
pg_upgrade.  Doing that causes an error to be raised unnecessarily:
MultiXactIdIsRunning is not open to the possibility that its argument
came from a pg_upgraded tuple, and all its other callers are already
checking; but such multis cannot, obviously, have transactions still
running, so the assert is pointless.

Noticed while investigating the bogus pg_multixact/offsets/0000 file
left over by pg_upgrade, as reported by Andres Freund in
http://www.postgresql.org/message-id/20140530121631.GE25431@alap3.anarazel.de

Backpatch to 9.3, as the commit that introduced the buglet.

src/backend/access/heap/heapam.c

@@ -5518,8 +5518,14 @@ FreezeMultiXactId(MultiXactId multi, uint16 t_infomask,
 		 * was a locker only, it can be removed without any further
 		 * consideration; but if it contained an update, we might need to
 		 * preserve it.
+		 *
+		 * Don't assert MultiXactIdIsRunning if the multi came from a
+		 * pg_upgrade'd share-locked tuple, though, as doing that causes an
+		 * error to be raised unnecessarily.
 		 */
-		Assert(!MultiXactIdIsRunning(multi));
+		Assert((!(t_infomask & HEAP_LOCK_MASK) &&
+				HEAP_XMAX_IS_LOCKED_ONLY(t_infomask)) ||
+			   !MultiXactIdIsRunning(multi));
 		if (HEAP_XMAX_IS_LOCKED_ONLY(t_infomask))
 		{
 			*flags |= FRM_INVALIDATE_XMAX;