nimble/host: Fix for BLE_HS_DBG_ASSERT in adv_set_fields
nimble/host: Fix for BLE_HS_DBG_ASSERT in adv_set_fields

This is regression after
95833796 nimble/host: Fix checking dst_len in adv_set_fields

Function ble_hs_adv_set_fields_mbuf() does not use dst_len which
triggers the BLE_HS_DBG_ASSERT

For that reason, we have now local variable in the adv_set_fields which
is later copied to dst_len if needed.

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble