BUG/MAJOR: connection: prevent double free if conn selected for removal

Always try to remove a connexion from its toremove_list in conn_free.
This prevents a double-free in case the connection is freed but was
already added in toremove_list.

This bug was easily reproduced by running 4-5 runs of inject on a
single-thread instance of haproxy :

$ inject -u 10000 -d 10 -G 127.0.0.1:20080

A crash would soon be triggered in srv_cleanup_toremove_connections.

This does not need to be backported.

include/haproxy/connection.h

@@ -504,6 +504,14 @@ static inline void conn_free(struct connection *conn)
 			srv_release_conn(__objt_server(conn->target), conn);
 	}
 
+	/* Remove the conn from toremove_list.
+	 *
+	 * This is needed to prevent a double-free in case the connection was
+	 * already scheduled from cleaning but is freed before via another
+	 * call.
+	 */
+	MT_LIST_DEL(&conn->toremove_list);
+
 	sockaddr_free(&conn->src);
 	sockaddr_free(&conn->dst);