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

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

Change-Id: I39a792a1fbe4639effe803ab09d2b90e254ca9c2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>