Fix OOB Write in pin_reply in bluetooth.cc

Root cause:
if the length of "pin_code" is greater than 16,
an OOBW will be triggered due to a missing bounds check.

Fix:
Check is added to avoid Out of Bound Write.

CRs-Fixed: 3507292
Change-Id: I15a1eae59b17f633e29180a01676c260189b8353