MESSAGE: Flush Message/ast_msg_queue channel alert pipe.

ASTERISK-25083

Change-Id: Id54baa57a8dbca84e29f28bcd2ffc0a5ac12d8b2

main/message.c

@@ -775,11 +775,20 @@ static void chan_cleanup(struct ast_channel *chan)
 	if (msg_ds) {
 		ast_channel_datastore_add(chan, msg_ds);
 	}
+
 	/*
 	 * Clear softhangup flags.
 	 */
 	ast_channel_clear_softhangup(chan, AST_SOFTHANGUP_ALL);
 
+	/*
+	 * Flush the alert pipe in case we miscounted somewhere when
+	 * messing with frames on the read queue, we had to flush the
+	 * read queue above, or we had an "Exceptionally long queue
+	 * length" event.
+	 */
+	ast_channel_internal_alert_flush(chan);
+
 	ast_channel_unlock(chan);
 }