[heap] Don't read current bucket twice when interating over slot set.

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..07a854dbb8babe7298552a10ae4d26e8556ec697 100644
--- a/src/heap/slot-set.h
+++ b/src/heap/slot-set.h
@@ -168,10 +168,10 @@ class SlotSet : public Malloced {
   int Iterate(Callback callback, EmptyBucketMode mode) {
     int new_count = 0;
     for (int bucket_index = 0; bucket_index < kBuckets; bucket_index++) {
-      if (bucket[bucket_index].Value() != nullptr) {
+      base::AtomicValue<uint32_t>* current_bucket =
+          bucket[bucket_index].Value();
+      if (current_bucket != nullptr) {
         int in_bucket_count = 0;
-        base::AtomicValue<uint32_t>* current_bucket =
-            bucket[bucket_index].Value();
         int cell_offset = bucket_index * kBitsPerBucket;
         for (int i = 0; i < kCellsPerBucket; i++, cell_offset += kBitsPerCell) {
           if (current_bucket[i].Value()) {