Add a log message to capture NULL pointer in l2c_link_hci_disc_comp()

Added a log message inside l2c_link_hci_disc_comp() to help capture
a NULL pointer p_lcb->p_fixed_ccbs[xx] that triggers SIGSEGV crash.
Also, updated the typedef of bdstr_t and moved it to
file "btcore/include/bdaddr.h"

Bug: 28765514
Change-Id: Iacd9376cfb9e18b58e18865581da8edad9f68821