device: Empty networks table before freeing device ident

Networks still need it when they are being deallocated:

  #0  connman_device_get_ident (src/device.c)
  #1  __connman_network_get_ident (src/network.c)
  sailfishos#2  connman_service_lookup_from_network (src/service.c)
  sailfishos#3  __connman_service_remove_from_network (src/service.c)
  sailfishos#4  network_remove (src/network.c)
  sailfishos#5  __connman_network_set_device (src/network.c)
  sailfishos#6  free_network (src/device.c)
  sailfishos#7  g_hash_table_remove_all_nodes (ghash.c)
  sailfishos#9  g_hash_table_remove_all (ghash.c)
  sailfishos#10 g_hash_table_destroy (ghash.c)
  sailfishos#11 device_destruct (src/device.c)

which results in use after free.

(cherry picked from commit 5e1daf5)