gboxed: Fix two potential NULL pointer dereferences
gboxed: Fix two potential NULL pointer dereferences

I don’t think these could be hit in practice due to the guarantees of
the type system, but the static analyser doesn’t know that — so make the
assertions clearer to shut it up.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #1767

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