Check the slots being adding to the SlotsBuffer.

Related to crbug/454297.

Review URL: https://codereview.chromium.org/897523003

Cr-Commit-Position: refs/heads/master@{#26410}

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;
   }