I'm not sure but I think that I've found a typo in do_split.
It seems that sep points to the program tree so it cannot be temp.
The fs variable, on the other hand, can be temp.

--- gawk-3.1.1/field.c.orig	Mon May 13 14:56:04 2002
+++ gawk-3.1.1/field.c	Mon May 13 14:55:17 2002
@@ -895,7 +895,7 @@ do_split(NODE *tree)
 					     fs, rp, set_element, arr));
 out:
 	unref(src);
-	free_temp(sep);
+	free_temp(fs);
 	return tmp;
 }