nmtui: Add a not reached assert to make compiler happy

nmt-editor.c: In function ‘nmt_editor_constructed’:
nmt-editor.c:365:2: error: ‘page’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  add_sections_for_page (editor, grid, page);
  ^
Merge remote-tracking branches 'fd/lr/ipv6-tokens', 'fd/lr/veth-managed' and 'fd/lr/no-udev' into integration