Add null check in AppendToBootClassPath

Aim to diagnose test 1002 that crashed once due to having a null
element in ClassLinker::boot_class_path_.

Bug: 144895912
Test: test/testrunner/test.py --host -j32

Change-Id: Ibc072dbc1bd48cfde0ad5d3b56ebdc348d441085
7 files changed