htsp server: fix race for bytes_out updating

htsp server: fix race for bytes_out updating

==12303== Invalid read of size 8
==12303==    at 0x455120: atomic_add_u64 (atomic.h:40)
==12303==    by 0x455120: subscription_add_bytes_out (subscriptions.c:1032)
==12303==    by 0x461524: htsp_stream_deliver (htsp_server.c:3702)
==12303==    by 0x461524: htsp_streaming_input (htsp_server.c:4085)
==12303==    by 0x507E53: _process_msg (timeshift_writer.c:340)
==12303==    by 0x507E53: timeshift_writer (timeshift_writer.c:395)
==12303==    by 0x439BB6: thread_wrapper (wrappers.c:177)
==12303==    by 0x61380A3: start_thread (pthread_create.c:309)
==12303==    by 0x693C04C: clone (clone.S:111)
==12303==  Address 0x1d04cc08 is 200 bytes inside a block of size 416 free'd
==12303==    at 0x4C29E90: free (vg_replace_malloc.c:473)
==12303==    by 0x454051: subscription_unsubscribe (subscriptions.c:647)
==12303==    by 0x4599F6: htsp_subscription_destroy (htsp_server.c:346)
==12303==    by 0x45C6FD: htsp_method_unsubscribe (htsp_server.c:2419)
==12303==    by 0x45E512: htsp_read_loop (htsp_server.c:3046)
==12303==    by 0x45E512: htsp_serve (htsp_server.c:3182)
==12303==    by 0x43D67C: tcp_server_start (tcp.c:644)
==12303==    by 0x439BB6: thread_wrapper (wrappers.c:177)
==12303==    by 0x61380A3: start_thread (pthread_create.c:309)
==12303==    by 0x693C04C: clone (clone.S:111)