nimble/hci: Fix passing uninitialized data

In ble_hs_hci_evt_le_dir_adv_rpt function desc.length_data was passed
uninitialized to ble_gap_rx_adv_report. While it shouldn't be access
if desc.data is NULL it is better to just initialize it to 0.

X-Original-Commit: 4e7d583466b6bd3917e984eb47d8b06f2e20c353

nimble/host/src/ble_hs_hci_evt.c

@@ -434,6 +434,7 @@ ble_hs_hci_evt_le_dir_adv_rpt(uint8_t subevent, uint8_t *data, int len)
 
     /* Data fields not present in a direct advertising report. */
     desc.data = NULL;
+    desc.length_data = 0;
 
     for (i = 0; i < num_reports; i++) {
         suboff = 0;