Fix the mutex lock when the memory allocation fails in api_listen_thread.

Thanks to John Jones for pointing out the problem.

core/api.c

@@ -538,6 +538,7 @@ void *api_listen_thread (void *ptr)
 		api_list.conns[i] = malloc (sizeof (struct s_conns));
 		if (!api_list.conns[i]) {
 			libp2p_logger_error("api", "Fail to allocate memory to accept connection.\n");
+			pthread_mutex_unlock(&conns_lock);
 			close (s);
 			continue;
 		}