fix add() method throw NPE

Signed-off-by: yuwei <yuwei@wesine.com>

redisson/src/main/java/org/redisson/RedissonSortedSet.java

@@ -47,7 +47,7 @@ public class RedissonSortedSet<V> extends RedissonObject implements RSortedSet<V
     public static class BinarySearchResult<V> {
 
         private V value;
-        private Integer index;
+        private int index = -1;
 
         public BinarySearchResult(V value) {
             super();