Address read barrier issue with cl/106467

And tidy/add a check.

Bug: 12687968
Change-Id: If63dc0d9d0a0ce5f2eeb81734ff8f4307865f67d
diff --git a/runtime/runtime.cc b/runtime/runtime.cc
index 89ad505..f258a30 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -809,6 +809,7 @@
 
   // Initialize the special sentinel_ value early.
   sentinel_ = GcRoot<mirror::Object>(class_linker_->AllocObject(self));
+  CHECK(sentinel_.Read() != nullptr);
 
   verifier::MethodVerifier::Init();