HAWQ-1770. fix memUse bug when reserve new block
diff --git a/depends/dbcommon/src/dbcommon/utils/flat-memory-buffer.h b/depends/dbcommon/src/dbcommon/utils/flat-memory-buffer.h
index df73629..73a86e3 100644
--- a/depends/dbcommon/src/dbcommon/utils/flat-memory-buffer.h
+++ b/depends/dbcommon/src/dbcommon/utils/flat-memory-buffer.h
@@ -101,6 +101,7 @@
       memBlkPtrListVec.push_back(newBlk.first);
       memBlkPtrList = &memBlkPtrListVec[0];
       memBlkList.push_back(std::move(newBlk.second));
+      lastBlkSize_ = MEMBLKSIZE;
     }
     assert(memBlkPtrListVec.size() == memBlkList.size());
     assert(memBlkList.size() >= filledBlockCount);