lib/db: Don't panic on unknown folder in ListFolders (fixes #3584)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3869

lib/db/leveldb_test.go

@@ -45,7 +45,10 @@ func TestGlobalKey(t *testing.T) {
 
 	key := db.globalKey(fld, name)
 
-	fld2 := db.globalKeyFolder(key)
+	fld2, ok := db.globalKeyFolder(key)
+	if !ok {
+		t.Error("should have been found")
+	}
 	if !bytes.Equal(fld2, fld) {
 		t.Errorf("wrong folder %q != %q", fld2, fld)
 	}
@@ -53,4 +56,9 @@ func TestGlobalKey(t *testing.T) {
 	if !bytes.Equal(name2, name) {
 		t.Errorf("wrong name %q != %q", name2, name)
 	}
+
+	_, ok = db.globalKeyFolder([]byte{1, 2, 3, 4, 5})
+	if ok {
+		t.Error("should not have been found")
+	}
 }