Merge: lib/core: fix `Text::is_int`

Fix #2019, `is_int` was always returning true. It was caused by the use of the wrong local variable.

Ping @R4PaSs and @Freddrickk.

Pull-Request: #2022
Reviewed-by: Frédéric Vachon <fredvac@gmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>