Do not visit null object from transaction logs.
Do not visit null object from transaction logs.

In debug build, it could cause check failure in MarkSweep::MarkObjectNonNull.

Change-Id: I840d1682e8150d7b04e83c7c25c340008dd82e70

treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst,treadstoneproject/artinst