[Cherry-Pick] Fix read wrong variable caused nil error (#22297)

Signed-off-by: NCongqi Xia <congqi.xia@zilliz.com>

internal/proxy/meta_cache.go

@@ -644,7 +644,7 @@ func (m *MetaCache) GetShards(ctx context.Context, withCache bool, collectionNam
 		info.leaderMutex.RUnlock()
 
 		if shardLeaders != nil {
-			iterator := info.shardLeaders.GetReader()
+			iterator := shardLeaders.GetReader()
 			return iterator.Shuffle(), nil
 		}