Check the slots being adding to the SlotsBuffer.

src/heap/mark-compact.h

Index: src/heap/mark-compact.h
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index 361f3a6377392022e69931832f2ee8ec6962d91d..50c23dd810d7d17552f5dcdd9392711a640b5065 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -266,6 +266,11 @@ class SlotsBuffer {
 
   void Add(ObjectSlot slot) {
     DCHECK(0 <= idx_ && idx_ < kNumberOfElements);
+#ifdef DEBUG
+    if (slot >= reinterpret_cast<ObjectSlot>(NUMBER_OF_SLOT_TYPES)) {
+      DCHECK_NOT_NULL(*slot);
+    }
+#endif
     slots_[idx_++] = slot;
   }