Fix[mqbblp::ClusterCatalog]: get d_clusters.size() under mutex

Signed-off-by: Evgeny Malygin <emalygin@bloomberg.net>

Author: 678098

src/groups/mqb/mqbblp/mqbblp_clustercatalog.cpp

@@ -778,10 +778,10 @@ void ClusterCatalog::getClusters(
     BSLS_ASSERT_SAFE(out);
 
     out->clear();
-    out->reserve(d_clusters.size());
 
     bslmt::LockGuard<bslmt::Mutex> guard(&d_mutex);  // d_mutex LOCK
 
+    out->reserve(d_clusters.size());
     for (ClustersMapConstIter it = d_clusters.begin(); it != d_clusters.end();
          ++it) {
         out->push_back(it->second.d_cluster_sp);