nimble/ll: silence compiler warning in ble_ll_iso_big_event_done

Silences error: 'hci_ev_ncp' may be used uninitialized: it is derived
from hci_ev, and only used when hci_ev != NULL. So it'll never be NULL
when used.

nimble/controller/src/ble_ll_iso_big.c

@@ -398,7 +398,7 @@ ble_ll_iso_big_event_done(struct ble_ll_iso_big *big)
 {
  struct ble_ll_iso_bis *bis;
  struct ble_hci_ev *hci_ev;
- struct ble_hci_ev_num_comp_pkts *hci_ev_ncp;
+ struct ble_hci_ev_num_comp_pkts *hci_ev_ncp = NULL;
  int num_completed_pkt;
  int rc;