Avoid crashing in GetOldestSnapshot() if there are no known snapshots.

The sole caller expects NULL to be returned in such a case, so make
it so and document it.

Per reports from Andreas Seltenreich and Regina Obe.  This doesn't
really fix their problem, as now their RETURNING queries will say
"ERROR: no known snapshots", but in any case this function should
not dump core in a reasonably-foreseeable situation.

Report: <87vazemeda.fsf@credativ.de>
Report: <20160807051854.1427.32414@wrigleys.postgresql.org>

src/backend/utils/time/snapmgr.c

@@ -399,14 +399,14 @@ GetLatestSnapshot(void)
 /*
  * GetOldestSnapshot
  *
- *		Get the oldest known snapshot, as judged by the LSN.
+ *		Get the transaction's oldest known snapshot, as judged by the LSN.
+ *		Will return NULL if there are no active or registered snapshots.
  */
 Snapshot
 GetOldestSnapshot(void)
 {
 	Snapshot	OldestRegisteredSnapshot = NULL;
 	XLogRecPtr	RegisteredLSN = InvalidXLogRecPtr;
-	XLogRecPtr	ActiveLSN = InvalidXLogRecPtr;
 
 	if (!pairingheap_is_empty(&RegisteredSnapshots))
 	{
@@ -416,10 +416,12 @@ GetOldestSnapshot(void)
 	}
 
 	if (OldestActiveSnapshot != NULL)
-		ActiveLSN = OldestActiveSnapshot->as_snap->lsn;
+	{
+		XLogRecPtr	ActiveLSN = OldestActiveSnapshot->as_snap->lsn;
 
 		if (XLogRecPtrIsInvalid(RegisteredLSN) || RegisteredLSN > ActiveLSN)
 			return OldestActiveSnapshot->as_snap;
+	}
 
 	return OldestRegisteredSnapshot;
 }