svc_chngd_ccc don't actually set a callback thus when cleaning up in
clear_ccc_state has to check if there is any callback set:

invalid address stated on the next line
  at 0x0: ???
  by 0x475C7C: clear_ccc_state (gatt-database.c:287)
  by 0x4D28CF: queue_foreach (queue.c:220)
  by 0x475FE7: att_disconnected (gatt-database.c:310)
  by 0x4D7255: disconn_handler (att.c:538)
  by 0x4D28CF: queue_foreach (queue.c:220)
  by 0x4D8F39: disconnect_cb (att.c:590)
  by 0x4E6B3A: watch_callback (io-glib.c:170)
  by 0x50CD246: g_main_context_dispatch (in /usr/lib64/libglib-2.0.so.0.5200.3)
  by 0x50CD5E7: ??? (in /usr/lib64/libglib-2.0.so.0.5200.3)
  by 0x50CD901: g_main_loop_run (in /usr/lib64/libglib-2.0.so.0.5200.3)
  by 0x40CD90: main (main.c:770)
Address 0x0 is not stack'd, malloc'd or (recently) free'd