Do not allow read barriers with null Thread::Current(). am: ad53509967 am: a33c24792c

Original change: https://android-review.googlesource.com/c/platform/art/+/2375888

Change-Id: I44b7a3e79d0288f2627d5ebe12c2eeb1fccab24b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>