Ignore WAL indices before snapshot that is being restored

If the replica has stray WAL files from older snapshots they will
break restoring even though they can be ignored and reaped later
through retention.

replica.go

@@ -1120,7 +1120,7 @@ func (r *Replica) Restore(ctx context.Context, opt RestoreOptions) (err error) {
 	}
 
 	// Compute list of offsets for each WAL index.
-	walSegmentMap, err := r.walSegmentMap(ctx, opt.Generation, opt.Index, opt.Timestamp)
+	walSegmentMap, err := r.walSegmentMap(ctx, opt.Generation, minWALIndex, opt.Index, opt.Timestamp)
 	if err != nil {
 		return fmt.Errorf("cannot find max wal index for restore: %w", err)
 	}
@@ -1328,7 +1328,7 @@ func (r *Replica) SnapshotIndexByIndex(ctx context.Context, generation string, i
 
 // walSegmentMap returns a map of WAL indices to their segments.
 // Filters by a max timestamp or a max index.
-func (r *Replica) walSegmentMap(ctx context.Context, generation string, maxIndex int, maxTimestamp time.Time) (map[int][]int64, error) {
+func (r *Replica) walSegmentMap(ctx context.Context, generation string, minIndex, maxIndex int, maxTimestamp time.Time) (map[int][]int64, error) {
 	itr, err := r.Client.WALSegments(ctx, generation)
 	if err != nil {
 		return nil, err
@@ -1344,6 +1344,8 @@ func (r *Replica) walSegmentMap(ctx context.Context, generation string, maxIndex
 			break // after max timestamp, skip
 		} else if info.Index > maxIndex {
 			break // after max index, skip
+		} else if info.Index < minIndex {
+			continue // ignore indices before snapshot
 		}
 
 		// Verify offsets are added in order.