cache: correctly ignore files whose filename is no ID

this can for example be the case for temporary files created by the
backend implementation.

internal/backend/cache/backend.go

@@ -231,9 +231,8 @@ func (b *Backend) List(ctx context.Context, t backend.FileType, fn func(f backen
 	wrapFn := func(f backend.FileInfo) error {
 		id, err := restic.ParseID(f.Name)
 		if err != nil {
-			// returning error here since, if we cannot parse the ID, the file
-			// is invalid and the list must exit.
-			return err
+			// ignore files with invalid name
+			return nil
 		}
 
 		ids.Insert(id)

internal/backend/cache/backend_test.go

@@ -296,3 +296,20 @@ func TestAutomaticCacheClear(t *testing.T) {
 		t.Errorf("cache doesn't have file2 after list")
 	}
 }
+
+func TestAutomaticCacheClearInvalidFilename(t *testing.T) {
+	be := mem.New()
+	c := TestNewCache(t)
+
+	data := test.Random(rand.Int(), 42)
+	h := backend.Handle{
+		Type: backend.IndexFile,
+		Name: "tmp12345",
+	}
+	save(t, be, h, data)
+
+	wbe := c.Wrap(be)
+
+	// list all files in the backend
+	list(t, wbe, func(_ backend.FileInfo) error { return nil })
+}