fix: minor fix in storage count implementation

Author: tglman

core/src/main/java/com/orientechnologies/orient/core/index/engine/v1/OCellBTreeMultiValueIndexEngine.java

@@ -527,7 +527,7 @@ private long mvTreeSize(final IndexEngineValuesTransformer transformer) {
       final Object firstKey = mvTree.firstKey();
       final Object lastKey = mvTree.lastKey();
 
-      int counter = 0;
+      long counter = 0;
 
       try (Stream<ORID> oridStream = mvTree.get(null)) {
         if (oridStream.iterator().hasNext()) {