nimble/host: Set length correctly in periodic_set_adv_data

   This change sets length field only if data is non NULL, else sets
   length to zero.

nimble/host/src/ble_gap.c

@@ -3711,9 +3711,12 @@ ble_gap_periodic_adv_set(uint8_t instance, struct os_mbuf **data)
  static uint8_t buf[sizeof(struct ble_hci_le_set_periodic_adv_data_cp) +
  MYNEWT_VAL(BLE_EXT_ADV_MAX_SIZE)];
  struct ble_hci_le_set_periodic_adv_data_cp *cmd = (void *) buf;
- uint16_t len = OS_MBUF_PKTLEN(*data);
+ uint16_t len = 0;
  uint16_t opcode;
 
+ if (*data)
+ len = OS_MBUF_PKTLEN(*data);
+
  opcode = BLE_HCI_OP(BLE_HCI_OGF_LE, BLE_HCI_OCF_LE_SET_PERIODIC_ADV_DATA);
 
  cmd->adv_handle = instance;
@@ -3730,11 +3733,14 @@ ble_gap_periodic_adv_set(uint8_t instance, struct os_mbuf **data)
  static uint8_t buf[sizeof(struct ble_hci_le_set_periodic_adv_data_cp) +
  BLE_HCI_MAX_PERIODIC_ADV_DATA_LEN];
  struct ble_hci_le_set_periodic_adv_data_cp *cmd = (void *) buf;
- uint16_t len = OS_MBUF_PKTLEN(*data);
+ uint16_t len = 0;
  uint16_t opcode;
  uint8_t op;
  int rc;
 
+ if (*data)
+ len = OS_MBUF_PKTLEN(*data);
+
  opcode = BLE_HCI_OP(BLE_HCI_OGF_LE, BLE_HCI_OCF_LE_SET_PERIODIC_ADV_DATA);
  cmd->adv_handle = instance;