Fix closing flushed channel

Fix reading from freed memory.
  Free at: channel_set_state(c, CS_DOWN)
  Read at: WALK_LIST2_DELSAFE(c, n, x, tab->channels, table_node)

==00:00:00:00.261 24718==
==00:00:09:31.755 24718== Invalid read of size 8
==00:00:09:31.755 24718==    at 0x4061BA: rt_prune_table (rt-table.c:1688)
==00:00:09:31.755 24718==    by 0x405D5E: rt_event (rt-table.c:1559)
==00:00:09:31.755 24718==    by 0x45D089: ev_run (event.c:85)
==00:00:09:31.755 24718==    by 0x45D158: ev_run_list (event.c:142)
==00:00:09:31.755 24718==    by 0x462814: io_loop (io.c:2412)
==00:00:09:31.755 24718==    by 0x468712: main (main.c:833)
==00:00:09:31.755 24718==  Address 0x5601538 is 136 bytes inside a block of size 304 free'd
==00:00:09:31.755 24718==    at 0x4C29D2A: free (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==00:00:09:31.755 24718==    by 0x46FF3E: rfree (resource.c:166)
==00:00:09:31.755 24718==    by 0x470309: mb_free (resource.c:415)
==00:00:09:31.755 24718==    by 0x406A6B: rt_unlock_table (rt-table.c:1921)
==00:00:09:31.755 24718==    by 0x40DAE3: channel_do_down (proto.c:297)
==00:00:09:31.755 24718==    by 0x40DD46: channel_set_state (proto.c:359)
==00:00:09:31.755 24718==    by 0x4061AD: rt_prune_table (rt-table.c:1692)
==00:00:09:31.755 24718==    by 0x405D5E: rt_event (rt-table.c:1559)
==00:00:09:31.755 24718==    by 0x45D089: ev_run (event.c:85)
==00:00:09:31.755 24718==    by 0x45D158: ev_run_list (event.c:142)
==00:00:09:31.755 24718==    by 0x462814: io_loop (io.c:2412)
==00:00:09:31.755 24718==    by 0x468712: main (main.c:833)
==00:00:09:31.755 24718==  Block was alloc'd at
==00:00:09:31.755 24718==    at 0x4C28C10: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==00:00:09:31.755 24718==    by 0x470FBC: bird_xmalloc (xmalloc.c:29)
==00:00:09:31.755 24718==    by 0x4701E6: mb_alloc (resource.c:339)
==00:00:09:31.755 24718==    by 0x406C29: rt_commit (rt-table.c:1977)
==00:00:09:31.755 24718==    by 0x45C36D: config_do_commit (conf.c:269)
==00:00:09:31.755 24718==    by 0x45C545: config_commit (conf.c:361)
==00:00:09:31.755 24718==    by 0x4686F9: main (main.c:822)
==00:00:09:31.755 24718==