[td-225] add checks.

src/tsdb/src/tsdbRead.c

@@ -1490,6 +1490,7 @@ static int32_t getDataBlocksInFilesImpl(STsdbQueryHandle* pQueryHandle, bool* ex
     return code;
   }
 
+  assert(pQueryHandle->pFileGroup != NULL);
   cur->slot = ASCENDING_TRAVERSE(pQueryHandle->order)? 0:pQueryHandle->numOfBlocks-1;
   cur->fid = pQueryHandle->pFileGroup->fileId;