Fix crash issue

Fix crash issue

0  0xaae6c8da in notify_io_destroy (data=0xac2cbcb0) at src/gatt-client.c:1698
1698            DBG("notify_io destroyed. length: %d", queue_length(client->chrc->notify_clients));
(gdb) info args
data = 0xac2cbcb0
(gdb) p client
$1 = (struct notify_client *) 0xac2cbcb0
(gdb) p *client
$2 = {chrc = 0x0, ref_count = 0, owner = 0x0, watch = 0, notify_id = 56}

$1 = (struct notify_client *) 0x55a1388d20
(gdb)
$2 = (struct notify_client *) 0x55a1388d20
(gdb) p *client
p *client
$3 = {chrc = 0x1, ref_count = 0, owner = 0x0, watch = 0, notify_id = 0}

Change-Id: Icb84e5995d7a2cd5ce1447d075a30ce1cc1a2f2f
Signed-off-by: Wootak Jung <wootak.jung@samsung.com>