gtk: fix NULL pointer dereference

gtk: fix NULL pointer dereference

`gtk_window_get_default_size()` claims width/height are optional-out
arguments, but defers to `gtk_window_get_remembered_size()` which
may dereference a NULL-pointer.

Since `gtk_window_get_remembered_size()` is only called by
`gtk_window_get_default_size()`, collapse it into the latter
and perform the NULL check there.