Initialize version sooner in VersionedMap::begin

RootSet.cpp

@@ -123,6 +123,11 @@ struct RootSet::Impl {
       firstToFree = firstToFree->next;
       safe_free(tmp, ThreadSafeHandle::Impl::sizeForCapacity(tmp->capacity));
     }
+#ifndef NDEBUG
+    assert(rootCount() > 0);
+    auto *h = handle.load(std::memory_order_relaxed);
+    assert(h->versions()[h->lastLeq(oldestVersion)] <= oldestVersion);
+#endif
   }
 
   const uint32_t *roots() const {