sekrit: make check_key more strict

bin/sekrit.sh

@@ -48,7 +48,11 @@ err()
 
 check_key()
 {
-	[ "$1" = "${1%/}" ] || err "Key can not end in a slash."
+	case $1 in
+	*/ | /* | ./* | */./* | */. | ../* | */../* | */..)
+		err "Key can not end in a slash."
+		;;
+	esac
 }
 
 to_number()