g_variant_get_child(): flatten-first logic on '&'

Copy the flatten-first logic from g_variant_get(), and for the same
reason: if the user is requesting a direct pointer access to a
tree-based child inside of argument to this function then that child
could disappear later.

Forcing serialisation means that the pointer will remain valid as long
as the passed-in instance exists, which is the usual expectation with
GVariant API.

https://bugzilla.gnome.org/show_bug.cgi?id=755374