[Vala] read_line_utf8, read_line_utf8_async length parameter can be null

[Michele Dionisio <michele dionisio gmail com>]

I have create 2 bug in the bug report, one on glib and one on vala

https://bugzilla.gnome.org/show_bug.cgi?id=783350
https://bugzilla.gnome.org/show_bug.cgi?id=783351

the main issue is for
   read_line_utf8_async
that for me has to have the same prototype of
   read_line_async

I have double check the glib code and for me length is nullable (also if it
is not documented)

regards
Michele