Null check for irk entry

When BT is turning off btm_ble_vendor control block is cleaned up.
So irk_entry which points to the first element of irk_list is NULL
Included a null check for irk_entry to prevent stack crash.

Bug 17399097

Change-Id: Ib129f7dd06f9db61cc54257aa4a563fe2c9f41f2