avoid talloc abort in simtrace2 frontend_append_script_env()

talloc requires a valid context for the allocation, we cannot use
env[i] as context.

Change-Id: I82b5e6b91b39d6f5c2a7492adb8c6193083e0d70
diff --git a/src/client/user_shell.c b/src/client/user_shell.c
index 40bcddd..9c804c6 100644
--- a/src/client/user_shell.c
+++ b/src/client/user_shell.c
@@ -54,9 +54,9 @@
 	return 0;
 }
 
-int frontend_append_script_env(struct bankd_client *bc, char **env, size_t max_env)
+int frontend_append_script_env(struct bankd_client *bc, char **env, int idx, size_t max_env)
 {
-	return 0;
+	return idx;
 }