lib: fix broken assert in FStream::opens

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
niti: add support partial for NativeFile::address_is_null

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
niti: implement Pointer:address_is_null as "intern" for niti executing niti

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge: lib: fix broken assert in FStream::opens

The old assert didn't work since the return was always `!= null` in Nit. Now we use instead the C value of `_file`.

Pull-Request: #403
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>