Merge pull request #5798 from taosdata/hotfix/TD-3757

[TD-3757]fix client crash issue

src/query/src/qExtbuffer.c

@@ -266,6 +266,7 @@ int32_t tExtMemBufferFlush(tExtMemBuffer *pMemBuffer) {
     size_t retVal = fwrite((char *)&(first->item), pMemBuffer->pageSize, 1, pMemBuffer->file);
     if (retVal <= 0) {  // failed to write to buffer, may be not enough space
       ret = TAOS_SYSTEM_ERROR(errno);
+      pMemBuffer->pHead = first;
       return ret;
     }