Address read barrier issue with cl/106467

And tidy/add a check.

Bug: 12687968
Change-Id: If63dc0d9d0a0ce5f2eeb81734ff8f4307865f67d
diff --git a/runtime/runtime-inl.h b/runtime/runtime-inl.h
index 8b632b2..fe05073 100644
--- a/runtime/runtime-inl.h
+++ b/runtime/runtime-inl.h
@@ -29,9 +29,7 @@
 
 inline mirror::Object* Runtime::GetClearedJniWeakGlobal() {
   mirror::Object* obj = sentinel_.Read();
-  if (obj == nullptr) {
-    LOG(ERROR) << "Failed to return cleared JNI weak global sentinel";
-  }
+  DCHECK(obj != nullptr);
   return obj;
 }