accounts_dialog_account_display_name_changed_cb: check that accounts_dialog_model_get_selected_settings didn't return NULL (#597416)
accounts_dialog_account_display_name_changed_cb: check that accounts_dialog_model_get_selected_settings didn't return NULL (#597416)

All the other callers of this function cope with setting being NULL so
this one should do as well.

GNOME/telepathy-account-widgets,GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets,Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets