[BUG] cgrates: empty session tag triggers undefined memcpy during dialog persistence

[kimaiden1984-boop]

OpenSIPS version

version: opensips 4.1.0-dev (x86_64/linux)
git revision: 0cab946

Describe the bug

The fourth cgrates_acc() argument (session tag) is optional. When it is omitted, the session tag is represented as { .s = NULL, .len = 0 }.

During dialog-variable persistence, cgr_dlg_onwrite() calls:

memcpy(p, s->tag.s, s->tag.len);

This passes a NULL source pointer to memcpy(), even though the length is zero. A Clang UBSan build reports undefined behavior.

To reproduce

Enable dialog persistence and call cgrates_acc() without its optional tag:

modparam("dialog", "db_mode", 1)
modparam("dialog", "db_flush_vals_profiles", 1)

route {
    create_dialog();
    cgrates_acc("cdr", "alice", "bob");
    t_relay();
}

Send an INVITE and have the downstream UAS return a tagged 200 response. When the confirmed dialog is persisted, UBSan reports:

modules/cgrates/cgrates_acc.c:791:14: runtime error:
null pointer passed as argument 2, which is declared to never be null

Using a non-empty fourth argument traverses the same persistence path without the diagnostic.

Expected behavior

An empty optional tag should be serialized without invoking memcpy() with a NULL pointer.

Proposed fix

- memcpy(p, s->tag.s, s->tag.len);
+ if (s->tag.len)
+     memcpy(p, s->tag.s, s->tag.len);

This does not change the serialized format: an empty tag still contributes zero payload bytes.