Fix a cleanup issue in dlgparam_destroy.

If a dialog box is destroyed by the program before the user has
pressed one of the result-delivering buttons - e.g. because the parent
window closes so the dialog is no longer relevant to anything anyway -
then dlgparam_destroy would never call the client code's provided
callback. That makes sense in terms of the callback wanting to _take
action_ based on the result of the dialog box, but it ignores the
possibility that the callback may simply need to free its own context
structure.

So now dlgparam_destroy always calls the client's callback, even if
the result it passes is negative (meaning 'the user never got round to
pressing any of the dialog-ending buttons'), and all the existing
client callbacks handle the negative-result case by doing nothing
except freeing any allocated memory they might have.

unix/gtkwin.c

@@ -4016,6 +4016,13 @@ static void after_change_settings_dialog(void *vctx, int retval)
 
     sfree(vctx); /* we've copied this already */
 
+    if (retval < 0) {
+        /* If the dialog box was aborted without giving a result
+         * (probably because the whole session window closed), we have
+         * nothing further to do. */
+        return;
+    }
+
     assert(lenof(ww) == NCFGCOLOURS);
 
     inst->reconfigure_dialog = NULL;