manager.c: Fix FRACK when doing CoreShowChannelMap in DEVMODE

If you run an AMI CoreShowChannelMap on a channel that isn't in a
bridge and you're in DEVMODE, you can get a FRACK because the
bridge id is empty.  We now simply return an empty list for that
request.

(cherry picked from commit 0778c95e37)

main/manager.c

@@ -6850,6 +6850,13 @@ static int action_coreshowchannelmap(struct mansession *s, const struct message
 		return 0;
 	}
 
+	if (ast_strlen_zero(channel_snapshot->bridge->id)) {
+		astman_send_listack(s, m, "Channel map will follow", "start");
+		astman_send_list_complete_start(s, m, "CoreShowChannelMapComplete", 0);
+		astman_send_list_complete_end(s);
+		return 0;
+	}
+
 	bridge_snapshot = ast_bridge_get_snapshot_by_uniqueid(channel_snapshot->bridge->id);
 	if (!bridge_snapshot) {
 		astman_send_listack(s, m, "Channel map will follow", "start");