afc: null-terminate choices string array

Fixes extra buttons appearing in the dialog.

The fact that we pass the array length doesn't guarantee that the
function we are calling will actually work fine with a non
null-terminated array; in fact it ultimately gets passed to the
autogenerated GDBus code, which expects it to be null-terminated,
causing memory access out of the array bounds, and into the
content-types string array.

https://bugzilla.gnome.org/show_bug.cgi?id=682496