rtpsession: iterate ssrcs copy when scheduling remaining nacks

[Copilot]

rtp_session_on_timeout's final g_hash_table_foreach walks the live sess->ssrcs table while calling schedule_remaining_nacks, which drops the session lock to issue an upcall. If another thread inserts a source via obtain_source during that window, GLib's version check trips on resume:

GLib-CRITICAL: g_hash_table_foreach: assertion 'version == hash_table->version' failed

Changes

• subprojects/gst-plugins-good/gst/rtpmanager/rtpsession.c — Apply the copy-then-iterate pattern already used by the update_source and generate_rtcp foreach sites earlier in the same function. clone_ssrcs_hashtable refs each source, so the copy stays valid across the unlock window inside schedule_remaining_nacks. No callback or locking semantics change.

RTP_SESSION_LOCK (sess);
table_copy = g_hash_table_new_full (NULL, NULL, NULL,
    (GDestroyNotify) g_object_unref);
g_hash_table_foreach (sess->ssrcs[sess->mask_idx],
    (GHFunc) clone_ssrcs_hashtable, table_copy);
g_hash_table_foreach (table_copy,
    (GHFunc) schedule_remaining_nacks, &data);
g_hash_table_destroy (table_copy);
RTP_SESSION_UNLOCK (sess);

Worth proposing upstream.