修复effective_io_concurrency取值大于1时分区表全局索引BitmapHeapscan报错的问题。

src/gausskernel/runtime/executor/nodeBitmapHeapscan.cpp

@@ -1309,10 +1309,10 @@ void BitmapHeapPrefetchNext(
             Oid oldOid = GPIGetCurrPartOid(node->gpi_scan);
             int2 oldBktId = cbi_get_current_bucketid(node->cbi_scan);
             Relation oldheap = NULL;
+            Relation prefetchRel = scan->rs_rd;
             
             while (node->prefetch_pages < node->prefetch_target) {
                 TBMIterateResult* tbmpre = tbm_iterate(*prefetch_iterator);
-                Relation prefetchRel = scan->rs_rd;
                 hpscan = (tbm_is_crossbucket(node->tbm) ? (HBktTblScanDesc)node->ss.ss_currentScanDesc : NULL);
 
                 if (tbmpre == NULL) {