Fallback to showing UID/GID if user/group lookup fails

If you mount a filesystem from another computer then UID/GID might
not be mapped to any user. This changes it so that lf shows the UID
and GID instead of nothing if user/group lookup fails.

os.go

@@ -199,17 +199,23 @@ func isHidden(f os.FileInfo, path string, hiddenfiles []string) bool {
 
 func userName(f os.FileInfo) string {
 	if stat, ok := f.Sys().(*syscall.Stat_t); ok {
-		if u, err := user.LookupId(fmt.Sprint(stat.Uid)); err == nil {
+		uid := fmt.Sprint(stat.Uid)
+		if u, err := user.LookupId(uid); err == nil {
 			return u.Username
+		} else {
+			return uid
 		}
 	}
 	return ""
 }
 
 func groupName(f os.FileInfo) string {
 	if stat, ok := f.Sys().(*syscall.Stat_t); ok {
-		if g, err := user.LookupGroupId(fmt.Sprint(stat.Gid)); err == nil {
+		gid := fmt.Sprint(stat.Gid)
+		if g, err := user.LookupGroupId(gid); err == nil {
 			return g.Name
+		} else {
+			return gid
 		}
 	}
 	return ""