ObjPtr<>-ify mirror::Object::As*(), fix stale refs in test.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 31113334
Change-Id: Ia04c4996f5cbfa1f26acbadcd747342c57b1aac3
diff --git a/runtime/entrypoints/entrypoint_utils.cc b/runtime/entrypoints/entrypoint_utils.cc
index 11365cb..ee2ab56 100644
--- a/runtime/entrypoints/entrypoint_utils.cc
+++ b/runtime/entrypoints/entrypoint_utils.cc
@@ -164,7 +164,7 @@
     ThrowNullPointerException("null array in FILL_ARRAY_DATA");
     return false;
   }
-  mirror::Array* array = obj->AsArray();
+  ObjPtr<mirror::Array> array = obj->AsArray();
   DCHECK(!array->IsObjectArray());
   if (UNLIKELY(static_cast<int32_t>(payload->element_count) > array->GetLength())) {
     Thread* self = Thread::Current();