gdbus: pass string length to hexencode()
gdbus: pass string length to hexencode()

This is necessary for it to be able to encode strings
with embedded NUL characters.

https://bugzilla.gnome.org/show_bug.cgi?id=794170

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib