shared/att: Fix possible crash on disconnect

If there are pending request while disconnecting they would be notified
but clients may endup being freed in the proccess which will then be
calling bt_att_cancel to cancal its requests causing the following
trace:

Invalid read of size 4
   at 0x1D894C: enable_ccc_callback (gatt-client.c:1627)
   by 0x1D247B: disc_att_send_op (att.c:417)
   by 0x1CCC17: queue_remove_all (queue.c:354)
   by 0x1D47B7: disconnect_cb (att.c:635)
   by 0x1E0707: watch_callback (io-glib.c:170)
   by 0x48E963B: g_main_context_dispatch (in /usr/lib/libglib-2.0.so.0.6400.4)
   by 0x48E9AC7: ??? (in /usr/lib/libglib-2.0.so.0.6400.4)
   by 0x48E9ECF: g_main_loop_run (in /usr/lib/libglib-2.0.so.0.6400.4)
   by 0x1E0E97: mainloop_run (mainloop-glib.c:79)
   by 0x1E13B3: mainloop_run_with_signal (mainloop-notify.c:201)
   by 0x12BC3B: main (main.c:770)
 Address 0x7d40a28 is 24 bytes inside a block of size 32 free'd
   at 0x484A2E0: free (vg_replace_malloc.c:540)
   by 0x1CCC17: queue_remove_all (queue.c:354)
   by 0x1CCC83: queue_destroy (queue.c:73)
   by 0x1D7DD7: bt_gatt_client_free (gatt-client.c:2209)
   by 0x16497B: batt_free (battery.c:77)
   by 0x16497B: batt_remove (battery.c:286)
   by 0x1A0013: service_remove (service.c:176)
   by 0x1A9B7B: device_remove_gatt_service (device.c:3691)
   by 0x1A9B7B: gatt_service_removed (device.c:3805)
   by 0x1CC90B: queue_foreach (queue.c:220)
   by 0x1DE27B: notify_service_changed.isra.0.part.0 (gatt-db.c:369)
   by 0x1DE387: notify_service_changed (gatt-db.c:361)
   by 0x1DE387: gatt_db_service_destroy (gatt-db.c:385)
   by 0x1DE3EF: gatt_db_remove_service (gatt-db.c:519)
   by 0x1D674F: discovery_op_complete (gatt-client.c:388)
   by 0x1D6877: discover_primary_cb (gatt-client.c:1260)
   by 0x1E220B: discovery_op_complete (gatt-helpers.c:628)
   by 0x1E249B: read_by_grp_type_cb (gatt-helpers.c:730)
   by 0x1D247B: disc_att_send_op (att.c:417)
   by 0x1CCC17: queue_remove_all (queue.c:354)
   by 0x1D47B7: disconnect_cb (att.c:635)