Fix a nullptr check failure in list_foreach

During reset, when btm_sec_cb.sec_dev_rec becomes null and
encyption events is sent to the host, nullptr check
failure will cause the stack to crash.

Bug: 324655049
Test: m com.android.btservices
Test: see aosp/2879087
Flag: EXEMPT, add null check
Change-Id: I33052b78c8989930c8c9dbdb49924e5c230266c3