archiver: Let saveTree report a canceled context as an error

If the context was canceled then saveTree might receive a treeID or not
depending on the timing. This could cause saveTree to incorrectly return
a nil treeID as valid. Fix this always returning an error when the
context was canceled in the meantime.

internal/archiver/archiver.go

@@ -178,6 +178,10 @@ func (arch *Archiver) saveTree(ctx context.Context, t *restic.Tree) (restic.ID,
 		s.TreeBlobs++
 		s.TreeSize += uint64(len(buf))
 	}
+	// The context was canceled in the meantime, res.ID() might be invalid
+	if ctx.Err() != nil {
+		return restic.ID{}, s, ctx.Err()
+	}
 	return res.ID(), s, nil
 }