Null check thread peers in JVMTI class callbacks

Early in startup it is possible that some callbacks will be called
with null thread arguments. We were failing DCHECKS by trying to add
references to these null objects.

Test: ./test.py --host -j40
Test: ./test/run-test --host --jvmti-trace-stress 001-HelloWorld
Change-Id: I2fdc454859ca5081b2db12643b4d24267035f7e8