Add missing debugger root visiting.

Bug: 13634574
Change-Id: I2a76f6c43f1d0ad1922f06deb40a71ff651129fd
diff --git a/runtime/gc/accounting/remembered_set.cc b/runtime/gc/accounting/remembered_set.cc
index afa5054..56f7caa 100644
--- a/runtime/gc/accounting/remembered_set.cc
+++ b/runtime/gc/accounting/remembered_set.cc
@@ -68,6 +68,7 @@
 
   void operator()(mirror::Object* obj, MemberOffset offset, bool /* is_static */) const
       SHARED_LOCKS_REQUIRED(Locks::mutator_lock_) {
+    DCHECK(obj != nullptr);
     mirror::HeapReference<mirror::Object>* ref_ptr = obj->GetFieldObjectReferenceAddr(offset);
     if (target_space_->HasAddress(ref_ptr->AsMirrorPtr())) {
       *contains_reference_to_target_space_ = true;