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

@@ -201,6 +201,8 @@ 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 {
 			return u.Username
+		} else {
+			return fmt.Sprintf("%d", stat.Uid)
 		}
 	}
 	return ""
@@ -210,6 +212,8 @@ 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 {
 			return g.Name
+		} else {
+			return fmt.Sprintf("%d", stat.Gid)
 		}
 	}
 	return ""