[heap] RemoveRange of SlotSet should not push nullptr for empty buckets on the pre-free stack.

src/heap/slot-set.h

Index: src/heap/slot-set.h
diff --git a/src/heap/slot-set.h b/src/heap/slot-set.h
index 5616bc8778518d6a4f2930dff456f95208823b6b..578eb42d1f7e645a8d1403f0862bc6e9bd8bfa18 100644
--- a/src/heap/slot-set.h
+++ b/src/heap/slot-set.h
@@ -81,10 +81,12 @@ class SlotSet : public Malloced {
   }
 
   void PreFreeEmptyBucket(int bucket_index) {
-    base::LockGuard<base::Mutex> guard(&to_be_freed_buckets_mutex_);
     base::AtomicValue<uint32_t>* bucket_ptr = bucket[bucket_index].Value();
-    to_be_freed_buckets_.push(bucket_ptr);
-    bucket[bucket_index].SetValue(nullptr);
+    if (bucket_ptr != nullptr) {
+      base::LockGuard<base::Mutex> guard(&to_be_freed_buckets_mutex_);
+      to_be_freed_buckets_.push(bucket_ptr);
+      bucket[bucket_index].SetValue(nullptr);
+    }
   }
 
   // The slot offsets specify a range of slots at addresses: