Catch errors when walking cache directories

cmd/restic/cmd_cache.go

@@ -152,10 +152,15 @@ func runCache(opts CacheOptions, gopts GlobalOptions, args []string) error {
 func dirSize(path string) (int64, error) {
 	var size int64
 	err := filepath.Walk(path, func(_ string, info os.FileInfo, err error) error {
+		if err != nil || info == nil {
+			return err
+		}
+
 		if !info.IsDir() {
 			size += info.Size()
 		}
-		return err
+
+		return nil
 	})
 	return size, err
 }