check: Fix crash of --read-data-subset=x% on empty repository

Rounding up to at least one pack file should only be done if at least a
pack file exists.

changelog/unreleased/issue-3296

@@ -0,0 +1,7 @@
+Bugfix: Fix crash of `check --read-data-subset=x%` run for an empty repository
+
+`check --read-data-subset=x%` crashed when run for an empty repository. This
+has been fixed.
+
+https://github.com/restic/restic/issues/3296
+https://github.com/restic/restic/pull/3309

cmd/restic/cmd_check.go

@@ -331,7 +331,7 @@ func selectPacksByBucket(allPacks map[restic.ID]int64, bucket, totalBuckets uint
 func selectRandomPacksByPercentage(allPacks map[restic.ID]int64, percentage float64) map[restic.ID]int64 {
 	packCount := len(allPacks)
 	packsToCheck := int(float64(packCount) * (percentage / 100.0))
-	if packsToCheck < 1 {
+	if packCount > 0 && packsToCheck < 1 {
 		packsToCheck = 1
 	}
 	timeNs := time.Now().UnixNano()

cmd/restic/cmd_check_test.go

@@ -122,3 +122,10 @@ func TestSelectRandomPacksByPercentage(t *testing.T) {
 		rtest.Assert(t, ok, "Expected input and output to be equal")
 	}
 }
+
+func TestSelectNoRandomPacksByPercentage(t *testing.T) {
+	// that the a repository without pack files works
+	var testPacks = make(map[restic.ID]int64)
+	selectedPacks := selectRandomPacksByPercentage(testPacks, 10.0)
+	rtest.Assert(t, len(selectedPacks) == 0, "Expected 0 selected packs")
+}