[genius] Add assert to silence warning

[Jiri (George) Lebl <jirka src gnome org>]

commit c5096cc3b6d7f3cef2def299354a00db253aa328
Author: Jiri (George) Lebl <jiri lebl gmail com>
Date:   Wed Oct 13 16:43:08 2021 -0500

    Add assert to silence warning

 src/eval.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/src/eval.c b/src/eval.c
index 498696ce..a4ee2f72 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -816,6 +816,7 @@ replacenode(GelETree *to, GelETree *from)
 {
        GelETree *next = to->any.next;
        freetree_full(to,TRUE,FALSE);
+       g_assert(from != NULL);
        memcpy(to,from,sizeof(GelETree));
 
 #ifdef MEM_DEBUG_FRIENDLY